31
2
Everything you ever
wanted to know about
statistics (well, sort of)
Figure 2.1
The face of
innocence …
but what are the
hands doing?
2.1.  What will this chapter tell me? 1
As a child grows, it becomes important for them to fit models to the world: to be able to
reliably predict what will happen in certain situations. This need to build models that accurately
reflect reality is an essential part of survival. According to my parents (conveniently
I have no memory of this at all), while at nursery school one model of the world that I was
particularly enthusiastic to try out was ‘If I get my penis out, it will be really funny’. No doubt
to my considerable disappointment, this model turned out to be a poor predictor of positive
outcomes. Thankfully for all concerned, I soon learnt that the model ‘If I get my penis out
at nursery school the teachers and mummy and daddy are going to be quite annoyed’ was
32 DISCOVERING STATISTICS USING SPSS
a better ‘fit’ of the observed data. Fitting models that accurately reflect the observed data is
important to establish whether a theory is true. You’ll be delighted to know that this chapter
is all about fitting statistical models (and not about my penis). We edge sneakily away
from the frying pan of research methods and trip accidentally into the fires of statistics hell.
We begin by discovering what a statistical model is by using the mean as a straightforward
example. We then see how we can use the properties of data to go beyond the data we have
collected and to draw inferences about the world at large. In a nutshell then, this chapter lays
the foundation for the whole of the rest of the book, so it’s quite important that you read it
or nothing that comes later will make any sense. Actually, a lot of what comes later probably
won’t make much sense anyway because I’ve written it, but there you go.
2.2.  Building statistical models 1
We saw in the previous chapter that scientists are interested in discovering something
about a phenomenon that we assume actually exists (a ‘real-world’ phenomenon).
These real-world phenomena can be anything from the behaviour of interest rates in
the economic market to the behaviour of undergraduates at the end-of-exam party.
Whatever the phenomenon we desire to explain, we collect data from the real world
to test our hypotheses about the phenomenon. Testing these hypotheses involves
building statistical models of the phenomenon of interest.
The reason for building statistical models of real-world data is best explained by
analogy. Imagine an engineer wishes to build a bridge across a river. That engineer
would be pretty daft if she just built any old bridge, because the chances are that it
would fall down. Instead, an engineer collects data from the real world: she looks
at bridges in the real world and sees what materials they are made from, what
structures they use and so on (she might even collect data about whether these
bridges are damaged). She then uses this information to construct a model. She
builds a scaled-down version of the real-world bridge because it is impractical, not to mention
expensive, to build the actual bridge itself. The model may differ from reality in several ways –
it will be smaller for a start – but the engineer will try to build a model that best fits the situation
of interest based on the data available. Once the model has been built, it can be used to
predict things about the real world: for example, the engineer might test whether the bridge
can withstand strong winds by placing the model in a wind tunnel. It seems obvious that it is
important that the model is an accurate representation of the real world. Social scientists do
much the same thing as engineers: they build models of real-world processes in an attempt to
predict how these processes operate under certain conditions (see Jane Superbrain Box 2.1
below). We don’t have direct access to the processes, so we collect data that represent the processes
and then use these data to build statistical models (we reduce the process to a statistical
model). We then use this statistical model to make predictions about the real-world phenomenon.
Just like the engineer, we want our models to be as accurate as possible so that we can be
confident that the predictions we make are also accurate. However, unlike engineers we don’t
have access to the real-world situation and so we can only ever infer things about psychological,
societal, biological or economic processes based upon the models we build. If we want our
inferences to be accurate then the statistical model we build must represent the data collected
(the observed data) as closely as possible. The degree to which a statistical model represents
the data collected is known as the fit of the model.
Figure 2.2 illustrates the kinds of models that an engineer might build to represent the
real-world bridge that she wants to create. The first model (a) is an excellent representation
of the real-world situation and is said to be a good fit (i.e. there are a few small differences but
the model is basically a very good replica of reality). If this model is used to make predictions
about the real world, then the engineer can be confident that these predictions will be very
accurate, because the model so closely resembles reality. So, if the model collapses in a strong
Why do we build
statistical models?
Why do we build
statistical models?
33CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
(a) Good Fit (b) Moderate Fit (c) Poor Fit
The Real World
Figure 2.2
Fitting models
to real-world
data (see text
for details)
wind, then there is a good chance that the real bridge would collapse also. The second model
(b) has some similarities to the real world: the model includes some of the basic structural
features, but there are some big differences from the real-world bridge (namely the absence
of one of the supporting towers). This is what we might term a moderate fit (i.e. there are
some differences between the model and the data but there are also some great similarities).
If the engineer uses this model to make predictions about the real world then these predictions
may be inaccurate and possibly catastrophic (e.g. the model predicts that the bridge
will collapse in a strong wind, causing the real bridge to be closed down, creating 100-mile
tailbacks with everyone stranded in the snow; all of which was unnecessary because the real
bridge was perfectly safe – the model was a bad representation of reality). We can have some
confidence, but not complete confidence, in predictions from this model. The final model
(c) is completely different to the real-world situation; it bears no structural similarities to
the real bridge and is a poor fit (in fact, it might more accurately be described as an abysmal
fit). As such, any predictions based on this model are likely to be completely inaccurate.
Extending this analogy to the social sciences we can say that it is important when we fit a
statistical model to a set of data that this model fits the data well. If our model is a poor fit of
the observed data then the predictions we make from it will be equally poor.
are identical systems based on linear models (Cohen,
1968), yet they have different names and, in psychology
at least, are used largely in different contexts due to historical
divisions in methodology (Cronbach, 1957).
A linear model is simply a model that is based upon
a straight line; this means that we are usually trying to
summarize our observed data in terms of a straight line.
Suppose we measured how many chapters of this book
a person had read, and then measured their spiritual
enrichment. We could represent these hypothetical data
in the form of a scatterplot in which each dot represents
an individual’s score on both variables (see section 4.8).
Figure 2.3 shows two versions of such a graph that summarizes
the pattern of these data with either a straight
As behavioural and social scientists, most of the models
that we use to describe data tend to be linear models. For
example, analysis of variance (ANOVA) and regression
JANE SUPERBRAIN 2.1
Types of statistical models 1
34 DISCOVERING STATISTICS USING SPSS
2.3.  Populations and samples 1
As researchers, we are interested in finding results that apply to an entire population of
people or things. For example, psychologists want to discover processes that occur in all
humans, biologists might be interested in processes that occur in all cells, economists want
to build models that apply to all salaries, and so on. A population can be very general (all
human beings) or very narrow (all male ginger cats called Bob). Usually, scientists strive to
infer things about general populations rather than narrow ones. For example, it’s not very
interesting to conclude that psychology students with brown hair who own a pet hamster
named George recover more quickly from sports injuries if the injury is massaged (unless,
like René Koning,1
you happen to be a psychology student with brown hair who has a pet
hamster named George). However, if we can conclude that everyone’s sports injuries are
aided by massage this finding has a much wider impact.
Scientists rarely, if ever, have access to every member of a population. Psychologists
cannot collect data from every human being and ecologists cannot observe every male ginger
cat called Bob. Therefore, we collect data from a small subset of the population (known as a
sample) and use these data to infer things about the population as a whole. The bridge-building
1
A brown-haired psychology student with a hamster called Sjors (Dutch for George, apparently), who, after
reading one of my web resources, emailed me to weaken my foolish belief that this is an obscure combination of
possibilities.
Figure 2.3
A scatterplot of
the same data
with a linear
model fitted
(left), and with a
non-linear model
fitted (right)
It’s possible that many scientific disciplines are progressing
in a biased way because most of the models
that we tend to fit are linear (mainly because books like
this tend to ignore more complex curvilinear models).
This could create a bias because most published scientific
studies are ones with statistically significant results
and there may be cases where a linear model has been
a poor fit of the data (and hence the paper was not published),
yet a non-linear model would have fitted the data
well. This is why it is useful to plot your data first: plots tell
you a great deal about what models should be applied
to data. If your plot seems to suggest a non-linear model
then investigate this possibility (which is easy for me to
say when I don’t include such techniques in this book!).
(left) or curved (right) line. These graphs illustrate how
we can fit different types of models to the same data.
In this case we can use a straight line to represent our
data and it shows that the more chapters a person reads,
the less their spiritual enrichment. However, we can also
use a curved line to summarize the data and this shows
that when most, or all, of the chapters have been read,
spiritual enrichment seems to increase slightly (presumably
because once the book is read everything suddenly
makes sense – yeah, as if!). Neither of the two types of
model is necessarily correct, but it will be the case that
one model fits the data better than another and this is
why when we use statistical models it is important for us
to assess how well a given model fits the data.
35CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
engineer cannot make a full-size model of the bridge she wants to build and so she builds a
small-scale model and tests this model under various conditions. From the results obtained from
the small-scale model the engineer infers things about how the full-sized bridge will respond.
The small-scale model may respond differently to a full-sized version of the bridge, but the
larger the model, the more likely it is to behave in the same way as the full-size bridge. This
metaphor can be extended to scientists. We never have access to the entire population (the realsize
bridge) and so we collect smaller samples (the scaled-down bridge) and use the behaviour
within the sample to infer things about the behaviour in the population. The bigger the sample,
the more likely it is to reflect the whole population. If we take several random samples from the
population, each of these samples will give us slightly different results. However, on average,
large samples should be fairly similar.
2.4.  Simple statistical models 1
2.4.1.   The mean: a very simple statistical model 1
One of the simplest models used in statistics is the mean, which we encountered in section
1.7.2.3. In Chapter 1 we briefly mentioned that the mean was a statistical model of the data
because it is a hypothetical value that doesn’t have to be a value that is actually observed in
the data. For example, if we took five statistics lecturers and measured the number of friends
that they had, we might find the following data: 1, 2, 3, 3 and 4. If we take the mean number
of friends, this can be calculated by adding the values we obtained, and dividing by the
number of values measured: (1 + 2 + 3 + 3 + 4)/5 = 2.6. Now, we know that it is impossible
to have 2.6 friends (unless you chop someone up with a chainsaw and befriend their
arm, which, frankly is probably not beyond your average statistics lecturer) so the mean value
is a hypothetical value. As such, the mean is a model created to summarize our data.
2.4.2.  Assessing the fit of the mean: sums of squares,
variance and standard deviations 1
With any statistical model we have to assess the fit (to return to our bridge analogy we need
to know how closely our model bridge resembles the real bridge that we want to build). With
most statistical models we can determine whether the model is accurate by looking at how different
our real data are from the model that we have created. The easiest way to do this is to
look at the difference between the data we observed and the model fitted. Figure 2.4 shows the
number of friends that each statistics lecturer had, and also the mean number that we calculated
earlier on. The line representing the mean can be thought of as our model, and the circles
are the observed data. The diagram also has a series of vertical lines that connect each observed
value to the mean value. These lines represent the deviance between the observed data and
our model and can be thought of as the error in the model. We can calculate the magnitude
of these deviances by simply subtracting the mean value (x)
from each of the observed values
(xi
).2
For example, lecturer 1 had only 1 friend (a glove puppet of an ostrich called Kevin) and
so the difference is x1
− x=
1 − 2.6 = −1.6. You might notice that the deviance is a negative
number, and this represents the fact that our model overestimates this lecturer’s popularity: it
2
The xi
simply refers to the observed score for the ith person (so, the i can be replaced with a number that represents
a particular individual). For these data: for lecturer 1, xi
= x1
= 1; for lecturer 3, xi
= x3
= 3; for lecturer 5,
xi
= x5
= 4.
36 DISCOVERING STATISTICS USING SPSS
predicts that he will have 2.6 friends yet in reality he has only 1 friend (bless him!). Now, how
can we use these deviances to estimate the accuracy of the model? One possibility is to add up
the deviances (this would give us an estimate of the total error). If we were to do this we would
find that (don’t be scared of the equations, we will work through them step by step – if you
need reminding of what the symbols mean there is a guide at the beginning of the book):
total error = sum of deviances
=
X
ðxi − xÞ = ð−1:6Þ + ð−0:6Þ + ð0:4Þ + ð0:4Þ + ð1:4Þ = 0
So, in effect the result tells us that there is no total error between our model and the
observed data, so the mean is a perfect representation of the data. Now, this clearly isn’t
true: there were errors but some of them were positive, some were negative and they have
simply cancelled each other out. It is clear that we need to avoid the problem of which
direction the error is in and one mathematical way to do this is to square each error,3
that
is multiply each error by itself. So, rather than calculating the sum of errors, we calculate
the sum of squared errors. In this example:
sum of squrared errors ðSSÞ =
X
ðxi − xÞðxi − xÞ
= ð−1:6Þ2
+ ð−0:6Þ2
+ ð0:4Þ2
+ ð0:4Þ2
+ ð1:4Þ2
= 2:56 + 0:36 + 0:16 + 0:16 + 1:96
= 5:20
The sum of squared errors (SS) is a good measure of the accuracy of our model. However,
it is fairly obvious that the sum of squared errors is dependent upon the amount of data
that has been collected – the more data points, the higher the SS. To overcome this problem
we calculate the average error by dividing the SS by the number of observations (N). If
we are interested only in the average error for the sample, then we can divide by N alone.
However, we are generally interested in using the error in the sample to estimate the error
in the population and so we divide the SS by the number of observations minus 1 (the
reason why is explained in Jane Superbrain Box 2.2 below). This measure is known as the
variance and is a measure that we will come across a great deal:
3
When you multiply a negative number by itself it becomes positive.
Figure 2.4
Graph showing
the difference
between the
observed number
of friends that
each statistics
lecturer had, and
the mean number
of friends
37CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
variance ðs2
Þ =
SS
N − 1
=
P
ðxi − xÞ2
N − 1
=
5:20
4
= 1:3
	 (2.1)
The variance is, therefore, the average error between the mean and the observations made
(and so is a measure of how well the model fits the actual data). There is one problem with
the variance as a measure: it gives us a measure in units squared (because we squared each
error in the calculation). In our example we would have to say that the average error in our
data (the variance) was 1.3 friends squared. It makes little enough sense to talk about 1.3
friends, but it makes even less to talk about friends squared! For this reason, we often take
the square root of the variance (which ensures that the measure of average error is in the
same units as the original measure). This measure is known as the standard deviation and is
simply the square root of the variance. In this example the standard deviation is:
s =
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
ðxi − xÞ2
N − 1
s
=
ffiffiffiffiffiffiffi
1:3
p
= 1:14
	 (2.2)
where they play – there is only one position left. Therefore
there are 14 degrees of freedom; that is, for 14 players you
have some degree of choice over where they play, but for 1
player you have no choice. The degrees of freedom is one
less than the number of players.
In statistical terms the degrees of freedom relate to the
number of observations that are free to vary. If we take
a sample of four observations from a population, then
these four scores are free to vary in any way (they can be
any value). However, if we then use this sample of four
observations to calculate the standard deviation of the
population, we have to use the mean of the sample as
an estimate of the population’s mean. Thus we hold one
parameter constant. Say that the mean of the sample was
10; then we assume that the population mean is 10 also
and we keep this value constant. With this parameter fixed,
can all four scores from our sample vary? The answer is
no, because to keep the mean constant only three values
are free to vary. For example, if the values in the sample
were 8, 9, 11, 12 (mean = 10) and we changed three of
these values to 7, 15 and 8, then the final value must be
10 to keep the mean constant. Therefore, if we hold one
parameter constant then the degrees of freedom must
be one less than the sample size. This fact explains why
when we use a sample to estimate the standard deviation
of a population, we have to divide the sums of squares by
N − 1 rather than N alone.
Degreesoffreedom(df)isaverydifficultconcepttoexplain.
I’ll begin with an analogy. Imagine you’re the manager of
a rugby team and you have a team sheet with 15 empty
slots relating to the positions on the playing field. There
is a standard formation in rugby and so each team has
15 specific positions that must be held constant for the
game to be played. When the first player arrives, you have
the choice of 15 positions in which to place this player. You
place his name in one of the slots and allocate him to a
position (e.g. scrum-half) and, therefore, one position on
the pitch is now occupied. When the next player arrives,
you have the choice of 14 positions but you still have the
freedom to choose which position this player is allocated.
However, as more players arrive, you will reach the point
at which 14 positions have been filled and the final player
arrives. With this player you have no freedom to choose
JANE SUPERBRAIN 2.2
Degrees of freedom 2
38 DISCOVERING STATISTICS USING SPSS
The sum of squares, variance and standard deviation are all, therefore, measures of the ‘fit’ (i.e.
how well the mean represents the data). Small standard deviations (relative to the value of the
mean itself) indicate that data points are close to the mean. A large standard deviation (relative
to the mean) indicates that the data points are distant from the mean (i.e. the mean is not an
accurate representation of the data). A standard deviation of 0 would mean that all of the scores
were the same. Figure 2.5 shows the overall ratings (on a 5-point scale) of two lecturers after each
of five different lectures. Both lecturers had an average rating of 2.6 out of 5 across the lectures.
However, the first lecturer had a standard deviation of 0.55 (relatively small compared to the
mean). It should be clear from the graph that ratings for this lecturer were consistently close to the
mean rating. There was a small fluctuation, but generally his lectures did not vary in popularity.
As such, the mean is an accurate representation of his ratings. The mean is a good fit of the data.
The second lecturer, however, had a standard deviation of 1.82 (relatively high compared to the
mean). The ratings for this lecturer are clearly more spread from the mean; that is, for some lectures
he received very high ratings, and for others his ratings were appalling. Therefore, the mean
is not such an accurate representation of his performance because there was a lot of variability in
the popularity of his lectures. The mean is a poor fit of the data. This illustration should hopefully
make clear why the standard deviation is a measure of how well the mean represents the data.
SELF-TEST In section 1.7.2.2 we came across some
data about the number of friends that 11 people had on
Facebook (22, 40, 53, 57, 93, 98, 103, 108, 116, 121,
252). We calculated the mean for these data as 96.64.
Now calculate the sums of squares, variance, and
standard deviation.
SELF-TEST Calculate these values again but
excluding the extreme score (252).
2.4.3.    Expressing the mean as a model 2
The discussion of means, sums of squares and variance may seem a side track from the initial
point about fitting statistical models, but it’s not: the mean is a simple statistical model
Figure 2.5
Graphs
illustrating data
that have the
same mean but
different standard
deviations
39CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
that can be fitted to data. What do I mean by this? Well, everything in statistics essentially
boils down to one equation:
outcomei = ðmodelÞ + errori 	 (2.3)
This just means that the data we observe can be predicted from the model we choose to
fit to the data plus some amount of error. When I say that the mean is a simple statistical
model, then all I mean is that we can replace the word ‘model’ with the word ‘mean’ in
that equation. If we return to our example involving the number of friends that statistics
lecturers have and look at lecturer 1, for example, we observed that they had one friend
and the mean of all lecturers was 2.6. So, the equation becomes:
outcomelecturer1 = X + εlecturer1
1 = 2:6 + εlecturer1
represents the data well then most of the scores will cluster
close to the mean and the resulting standard deviation
is small relative to the mean. When the mean is a
worse representation of the data, the scores cluster more
widely around the mean (think back to Figure 2.5) and
the standard deviation is larger. Figure 2.6 shows two
distributions that have the same mean (50) but different
standard deviations. One has a large standard deviation
relative to the mean (SD = 25) and this results in a flatter
distribution that is more spread out, whereas the other
has a small standard deviation relative to the mean (SD =
15) resulting in a more pointy distribution in which scores
close to the mean are very frequent but scores further
from the mean become increasingly infrequent. The main
message is that as the standard deviation gets larger, the
distribution gets fatter. This can make distributions look
platykurtic or leptokurtic when, in fact, they are not.
As well as telling us about the accuracy of the mean
as a model of our data set, the variance and standard
deviation also tell us about the shape of the distribution
of scores. As such, they are measures of dispersion
like those we encountered in section 1.7.3. If the mean
JANE SUPERBRAIN 2.3
The standard deviation and
the shape of the distribution 1
Figure 2.6  Two distributions with the same mean, but large and small standard deviations
40 DISCOVERING STATISTICS USING SPSS
From this we can work out that the error is 1 – 2.6, or -1.6. If we replace this value
in the equation we get 1 = 2.6 – 1.6 or 1 = 1. Although it probably seems like I’m
stating the obvious, it is worth bearing this general equation in mind throughout this
book because if you do you’ll discover that most things ultimately boil down to this
one simple idea!
Likewise, the variance and standard deviation illustrate another fundamental concept:
how the goodness of fit of a model can be measured. If we’re looking at how well a
model fits the data (in this case our model is the mean) then we generally look at deviation
from the model, we look at the sum of squared error, and in general terms we can
write this as:
deviation =
X
ðobserved − modelÞ2
	
(2.4)
Put another way, we assess models by comparing the data we observe to the model we’ve
fitted to the data, and then square these differences. Again, you’ll come across this fundamental
idea time and time again throughout this book.
2.5.  Going beyond the data 1
Using the example of the mean, we have looked at how we can fit a statistical model to
a set of observations to summarize those data. It’s one thing to summarize the data that
you have actually collected but usually we want to go beyond our data and say something
general about the world (remember in Chapter 1 that I talked about how good theories
should say something about the world). It is one thing to be able to say that people in our
sample responded well to medication, or that a sample of high-street stores in Brighton
had increased profits leading up to Christmas, but it’s more useful to be able to say, based
on our sample, that all people will respond to medication, or that all high-street stores in
the UK will show increased profits. To begin to understand how we can make these general
inferences from a sample of data we can first look not at whether our model is a good fit
of the sample from which it came, but whether it is a good fit of the population from which
the sample came.
2.5.1.    The standard error 1
We’ve seen that the standard deviation tells us something about how well the mean
represents the sample data, but I mentioned earlier on that usually we collect data from
samples because we don’t have access to the entire population. If you take several samples
from a population, then these samples will differ slightly; therefore, it’s also important
to know how well a particular sample represents the population. This is where we use
the standard error. Many students get confused about the difference between the standard
deviation and the standard error (usually because the difference is never explained
clearly). However, the standard error is an important concept to grasp, so I’ll do my best
to explain it to you.
We have already learnt that social scientists use samples as a way of estimating the
behaviour in a population. Imagine that we were interested in the ratings of all lecturers
(so, lecturers in general were the population). We could take a sample from this population.
When someone takes a sample from a population, they are taking one of many
41CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
possible samples. If we were to take several samples from the same population, then each
sample has its own mean, and some of these sample means will be different.
Figure 2.7 illustrates the process of taking samples from a population. Imagine that we
could get ratings of all lecturers on the planet and that, on average, the rating is 3 (this
is the population mean, µ). Of course, we can’t collect ratings of all lecturers, so we use
a sample. For each of these samples we can calculate the average, or sample mean. Let’s
imagine we took nine different samples (as in the diagram); you can see that some of the
samples have the same mean as the population but some have different means: the first
sample of lecturers were rated, on average, as 3, but the second sample were, on average,
Figure 2.7
Illustration of the
standard error
(see text for
details)
42 DISCOVERING STATISTICS USING SPSS
rated as only 2. This illustrates sampling variation: that is, samples will vary because they
contain different members of the population; a sample that by chance includes some very
good lecturers will have a higher average than a sample that, by chance, includes some
awful lecturers. We can actually plot the sample means as a frequency distribution, or
histogram,4
just like I have done in the diagram. This distribution shows that there were
three samples that had a mean of 3, means of 2 and 4 occurred in two samples each, and
means of 1 and 5 occurred in only one sample each. The end result is a nice symmetrical
distribution known as a sampling distribution. A sampling distribution is simply the
frequency distribution of sample means from the same population. In theory you need
to imagine that we’re taking hundreds or thousands of samples to construct a sampling
distribution, but I’m just using nine to keep the diagram simple.5
The sampling distribution
tells us about the behaviour of samples from the population, and you’ll notice that it
is centred at the same value as the mean of the population (i.e. 3). This means that if we
took the average of all sample means we’d get the value of the population mean. Now,
if the average of the sample means is the same value as the population mean, then if we
know the accuracy of that average we’d know something about how likely it is that a
given sample is representative of the population. So how do we determine the accuracy
of the population mean?
Think back to the discussion of the standard deviation. We used the standard deviation
as a measure of how representative the mean was of the observed data. Small standard
deviations represented a scenario in which most data points were close to the mean, a
large standard deviation represented a situation in which data points were widely spread
from the mean. If you were to calculate the standard deviation between sample means
then this too would give you a measure of how much variability there was between the
means of different samples. The standard deviation of sample means is known as the
standard error of the mean (SE). Therefore, the standard error could be calculated by
taking the difference between each sample mean and the overall mean, squaring these
differences, adding them up, and then dividing by the number of samples. Finally, the
square root of this value would need to be taken to get the standard deviation of sample
means, the standard error.
Of course, in reality we cannot collect hundreds of samples and so we rely on approximations
of the standard error. Luckily for us some exceptionally clever statisticians have
demonstrated that as samples get large (usually defined as greater than 30), the sampling
distribution has a normal distribution with a mean equal to the population mean, and a
standard deviation of:
σX =
s
ffiffiffiffiffi
N
p
	 (2.5)
This is known as the central limit theorem and it is useful in this context because it means
that if our sample is large we can use the above equation to approximate the standard error
(because, remember, it is the standard deviation of the sampling distribution).6
When the
sample is relatively small (fewer than 30) the sampling distribution has a different shape,
known as a t-distribution, which we’ll come back to later.
4
This is just a graph of each sample mean plotted against the number of samples that has that mean – see section
1.7.1 for more details.
5
It’s worth pointing out that I’m talking hypothetically. We don’t need to actually collect these samples because
clever statisticians have worked out what these sampling distributions would look like and how they behave.
6
In fact it should be the population standard deviation (σ) that is divided by the square root of the sample size;
however, for large samples this is a reasonable approximation.
43CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
2.5.2.   Confidence intervals 2
2.5.2.1.  Calculating confidence intervals 2
Remember that usually we’re interested in using the sample mean as an estimate of the
value in the population. We’ve just seen that different samples will give rise to different values
of the mean, and we can use the standard error to get some idea of the extent to which
sample means differ. A different approach to assessing the accuracy of the sample mean
as an estimate of the mean in the population is to calculate boundaries within which we
believe the true value of the mean will fall. Such boundaries are called confidence intervals.
The basic idea behind confidence intervals is to construct a range of values within which
we think the population value falls.
Let’s imagine an example: Domjan, Blesbois, and Williams (1998) examined the learnt
release of sperm in Japanese quail. The basic idea is that if a quail is allowed to copulate
with a female quail in a certain context (an experimental chamber) then this context will
serve as a cue to copulation and this in turn will affect semen release (although during the
test phase the poor quail were tricked into copulating with a terry cloth with an embalmed
female quail head stuck on top)7
. Anyway, if we look at the mean amount of sperm released
in the experimental chamber, there is a true mean (the mean in the population); let’s imagine
it’s 15 million sperm. Now, in our actual sample, we might find the mean amount of
sperm released was 17 million. Because we don’t know the true mean, we don’t really know
whether our sample value of 17 million is a good or bad estimate of this value. What we can
do instead is use an interval estimate: we use our sample value as the mid-point, but set a
lower and upper limit as well. So, we might say, we think the true value of the mean sperm
release is somewhere between 12 million and 22 million spermatozoa (note that
17 million falls exactly between these values). Of course, in this case the true
value (15 million) does fall within these limits. However, what if we’d set
smaller limits, what if we’d said we think the true value falls between 16 and 18
million (again, note that 17 million is in the middle)? In this case the interval
does not contain the true value of the mean. Let’s now imagine that you were
particularly fixated with Japanese quail sperm, and you repeated the experiment
50 times using different samples. Each time you did the experiment again
you constructed an interval around the sample mean as I’ve just described.
Figure 2.8 shows this scenario: the circles represent the mean for each sample
with the lines sticking out from them representing the intervals for these means.
The true value of the mean (the mean in the population) is 15 million and is shown by a
vertical line. The first thing to note is that most of the sample means are different from
7
This may seem a bit sick, but the male quails didn’t appear to mind too much, which probably tells us all we
need to know about male mating behaviour.
CRAMMING SAM’s Tips The standard error
	 The standard error is the standard deviation of sample means. As such, it is a measure of how representative
a sample is likely to be of the population. A large standard error (relative to the sample mean) means that there is a lot of variability
between the means of different samples and so the sample we have might not be representative of the population. A small
standard error indicates that most sample means are similar to the population mean and so our sample is likely to be an accurate
reflection of the population.
What is a confidence
interval?
44 DISCOVERING STATISTICS USING SPSS
the true mean (this is because of sampling variation as described in the previous section).
Second, although most of the intervals do contain the true mean (they cross the vertical
line, meaning that the value of 15 million spermatozoa falls somewhere between the lower
and upper boundaries), a few do not.
Up until now I’ve avoided the issue of how we might calculate the intervals. The crucial
thing with confidence intervals is to construct them in such a way that they tell us something
useful. Therefore, we calculate them so that they have certain properties: in particular they
tell us the likelihood that they contain the true value of the thing we’re trying to estimate (in
this case, the mean).
Typically we look at 95% confidence intervals, and sometimes 99% confidence intervals,
but they all have a similar interpretation: they are limits constructed such that for
Figure 2.8
The confidence
intervals of the
sperm counts
of Japanese
quail (horizontal
axis) for
50 different
samples
(vertical axis)
45CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
a certain percentage of the time (be that 95% or 99%) the true value of the population
mean will fall within these limits. So, when you see a 95% confidence interval for a
mean, think of it like this: if we’d collected 100 samples, calculated the mean and then
calculated a confidence interval for that mean (a bit like in Figure 2.8) then for 95 of
these samples, the confidence intervals we constructed would contain the true value of
the mean in the population.
To calculate the confidence interval, we need to know the limits within which 95% of
means will fall. How do we calculate these limits? Remember back in section 1.7.4 that I
said that 1.96 was an important value of z (a score from a normal distribution with a mean
of 0 and standard deviation of 1) because 95% of z-scores fall between –1.96 and 1.96.
This means that if our sample means were normally distributed with a mean of 0 and a
standard error of 1, then the limits of our confidence interval would be –1.96 and +1.96.
Luckily we know from the central limit theorem that in large samples (above about 30) the
sampling distribution will be normally distributed (see section 2.5.1). It’s a pity then that
our mean and standard deviation are unlikely to be 0 and 1; except not really because, as
you might remember, we can convert scores so that they do have a mean of 0 and standard
deviation of 1 (z-scores) using equation (1.2):
z =
X − X
s
If we know that our limits are –1.96 and 1.96 in z­-scores, then to find out the corresponding
scores in our raw data we can replace z in the equation (because there are two values,
we get two equations):
1:96 =
X − X
s
−1:96 =
X − X
s
We rearrange these equations to discover the value of X:
1:96 × s = X − X −1:96 × s = X − X
ð1:96 × sÞ + X = X ð−1:96 × sÞ + X = X
Therefore, the confidence interval can easily be calculated once the standard deviation (s in
the equation above) and mean (X
in
the equation) are known. However, in fact we use the
standard error and not the standard deviation because we’re interested in the variability of
sample means, not the variability in observations within the sample. The lower boundary
of the confidence interval is, therefore, the mean minus 1.96 times the standard error, and
the upper boundary is the mean plus 1.96 standard errors.
lower boundary of confidence interval = X − ð1:96 × SEÞ
upper boundary of confidence interval = X + ð1:96 × SEÞ
As such, the mean is always in the centre of the confidence interval. If the mean represents
the true mean well, then the confidence interval of that mean should be small.
We know that 95% of confidence intervals contain the true mean, so we can assume this
confidence interval contains the true mean; therefore, if the interval is small, the sample
mean must be very close to the true mean. Conversely, if the confidence interval is very
wide then the sample mean could be very different from the true mean, indicating that it
is a bad representation of the population You’ll find that confidence intervals will come up
time and time again throughout this book.
46 DISCOVERING STATISTICS USING SPSS
2.5.2.2.  Calculating other confidence intervals 2
The example above shows how to compute a 95% confidence interval (the most common
type). However, we sometimes want to calculate other types of confidence interval such as
a 99% or 90% interval. The 1.96 and -1.96 in the equations above are the limits within
which 95% of z­-scores occur. Therefore, if we wanted a 99% confidence interval we could
use the values within which 99% of z-scores occur (-2.58 and 2.58). In general then, we
could say that confidence intervals are calculated as:
lower boundary of confidence interval = X − z1 − p
2
× SE
 
upper boundary of confidence interval = X + z1 − p
2
× SE
 
in which p is the probability value for the confidence interval. So, if you want a 95%
confidence interval, then you want the value of z for (1 - 0.95)/2 = 0.025. Look this up
in the ‘smaller portion’ column of the table of the standard normal distribution (see the
Appendix) and you’ll find that z is 1.96. For a 99% confidence interval we want z for (1 –
0.99)/2 = 0.005, which from the table is 2.58. For a 90% confidence interval we want z for
(1 – 0.90)/2 = 0.05, which from the table is 1.65. These values of z are multiplied by the
standard error (as above) to calculate the confidence interval. Using these general principles
we could work out a confidence interval for any level of probability that takes our fancy.
2.5.2.3.  Calculating confidence intervals in small samples 2
The procedure that I have just described is fine when samples are large, but for small
samples, as I have mentioned before, the sampling distribution is not normal, it has a
t-distribution. The t-distribution is a family of probability distributions that change shape
as the sample size gets bigger (when the sample is very big, it has the shape of a normal distribution).
To construct a confidence interval in a small sample we use the same principle
as before but instead of using the value for z we use the value for t:
lower boundary of confidence interval = X − ðtn − 1 × SEÞ
upper boundary of confidence interval = X + ðtn − 1 × SEÞ
The n – 1 in the equations is the degrees of freedom (see Jane Superbrain Box 2.3) and tells
us which of the t-distributions to use. For a 95% confidence interval we find the value of t
for a two-tailed test with probability of .05, for the appropriate degrees of freedom.
SELF-TEST In section 1.7.2.2 we came across some
data about the number of friends that 11 people had on
Facebook. We calculated the mean for these data as
96.64 and standard deviation as 61.27. Calculate a
95% confidence interval for this mean.
SELF-TEST Recalculate the confidence interval
assuming that the sample size was 56
47CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
2.5.2.4.  Showing confidence intervals visually 2
Confidence intervals provide us with very important information about the mean, and, therefore,
you often see them displayed on graphs. (We will discover more about how to create these graphs
in Chapter 4.) The confidence interval is usually displayed using something called an
error bar, which just looks like the letter ‘I’. An error bar can represent the standard
deviation, or the standard error, but more often than not it shows the 95% confidence
interval of the mean. So, often when you see a graph showing the mean, perhaps displayed
as a bar (section 4.6) or a symbol (section 4.7), it is often accompanied by this
funny I-shaped bar. Why is it useful to see the confidence interval visually?
We have seen that the 95% confidence interval is an interval constructed such that
in 95% of samples the true value of the population mean will fall within its limits. We
know that it is possible that any two samples could have slightly different means (and
the standard error tells us a little about how different we can expect sample means
to be). Now, the confidence interval tells us the limits within which the population
mean is likely to fall (the size of the confidence interval will depend on the size of the standard
error). By comparing the confidence intervals of different means we can start to get some idea
about whether the means came from the same population or different populations.
Taking our previous example of quail sperm, imagine we had a sample of quail and
the mean sperm release had been 9 million sperm with a confidence interval of 2 to 16.
Therefore, we know that the population mean is probably between 2 and 16 million sperm.
What if we now took a second sample of quail and found the confidence interval ranged
from 4 to 15? This interval overlaps a lot with our first sample:
The fact that the confidence intervals overlap in this way tells us that these means could
plausibly come from the same population: in both cases the intervals are likely to contain
the true value of the mean (because they are constructed such that in 95% of studies they
will), and both intervals overlap considerably, so they contain many similar values. What if
the confidence interval for our second sample ranges from 18 to 28? If we compared this
to our first sample we’d get:
Now, these confidence intervals don’t overlap at all. So, one confidence interval, which
is likely to contain the population mean, tells us that the population mean is somewhere
What’s an error bar?What’s an error bar?
48 DISCOVERING STATISTICS USING SPSS
between 2 and 16 million, whereas the other confidence interval, which is also likely to
contain the population mean, tells us that the population mean is somewhere between 18
and 28. This suggests that either our confidence intervals both do contain the population
mean, but they come from different populations (and, therefore, so do our samples), or
both samples come from the same population but one of the confidence intervals doesn’t
contain the population mean. If we’ve used 95% confidence intervals then we know that
the second possibility is unlikely (this happens only 5 times in 100 or 5% of the time), so
the first explanation is more plausible.
OK, I can hear you all thinking ‘so what if the samples come from a different population?’
Well, it has a very important implication in experimental research. When we do an
experiment, we introduce some form of manipulation between two or more conditions
(see section 1.6.2). If we have taken two random samples of people, and we have tested
them on some measure (e.g. fear of statistics textbooks), then we expect these people to
belong to the same population. If their sample means are so different as to suggest that,
in fact, they come from different populations, why might this be? The answer is that our
experimental manipulation has induced a difference between the samples.
To reiterate, when an experimental manipulation is successful, we expect to find that our
samples have come from different populations. If the manipulation is unsuccessful, then we
expect to find that the samples came from the same population (e.g. the sample means should
be fairly similar). Now, the 95% confidence interval tells us something about the likely value of
the population mean. If we take samples from two populations, then we expect the confidence
intervals to be different (in fact, to be sure that the samples were from different populations we
would not expect the two confidence intervals to overlap). If we take two samples from the same
population, then we expect, if our measure is reliable, the confidence intervals to be very similar
(i.e. they should overlap completely with each other).
This is why error bars showing 95% confidence intervals are so useful on graphs, because
if the bars of any two means do not overlap then we can infer that these means are from
different populations – they are significantly different.
CRAMMING SAM’s Tips Confidence intervals
	 A confidence interval for the mean is a range of scores constructed such that the population mean will fall within
this range in 95% of samples.
The confidence interval is not an interval within which we are 95% confident that the population mean will fall.
2.6.  Using statistical models to test research
questions 1
In Chapter 1 we saw that research was a five-stage process:
Generate a research question through an initial observation (hopefully backed up by1	
some data).
Generate a theory to explain your initial observation.2	
Generate hypotheses: break your theory down into a set of testable predictions.3	
49CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
Collect data to test the theory: decide on what variables you need to measure to test4	
your predictions and how best to measure or manipulate those variables.
Analyse the data: fit a statistical model to the data – this model will test your ori­5	
ginal predictions. Assess this model to see whether or not it supports your initial
predictions.
This chapter has shown us that we can use a sample of data to estimate what’s happening
in a larger population to which we don’t have access. We have also seen (using
the mean as an example) that we can fit a statistical model to a sample of data and
assess how well it fits. However, we have yet to see how fitting models like these can
help us to test our research predictions. How do statistical models help us to test complex
hypotheses such as ‘is there a relationship between the amount of gibberish that
people speak and the amount of vodka jelly they’ve eaten?’, or ‘is the mean amount
of chocolate I eat higher when I’m writing statistics books than when I’m not?’ We’ve
seen in section 1.7.5 that hypotheses can be broken down into a null hypothesis and
an alternative hypothesis.
SELF-TEST What are the null and alternative
hypotheses for the following questions:
ü	 ‘Is there a relationship between the amount of
gibberish that people speak and the amount of
vodka jelly they’ve eaten?’
ü	 ‘Is the mean amount of chocolate eaten higher
when writing statistics books than when not?’
Most of this book deals with inferential statistics, which tell us whether the alternative
hypothesis is likely to be true – they help us to confirm or reject our predictions. Crudely
put, we fit a statistical model to our data that represents the alternative hypothesis and see
how well it fits (in terms of the variance it explains). If it fits the data well (i.e. explains
a lot of the variation in scores) then we assume our initial prediction is true: we gain
confidence in the alternative hypothesis. Of course, we can never be completely sure that
either hypothesis is correct, and so we calculate the probability that our model would fit if
there were no effect in the population (i.e. the null hypothesis is true). As this probability
decreases, we gain greater confidence that the alternative hypothesis is actually correct and
that the null hypothesis can be rejected. This works provided we make our predictions
before we collect the data (see Jane Superbrain Box 2.4).
To illustrate this idea of whether a hypothesis is likely, Fisher (1925/1991) (Figure 2.9)
describes an experiment designed to test a claim by a woman that she could determine, by
tasting a cup of tea, whether the milk or the tea was added first to the cup. Fisher thought
that he should give the woman some cups of tea, some of which had the milk added first
and some of which had the milk added last, and see whether she could correctly identify
them. The woman would know that there are an equal number of cups in which milk was
added first or last but wouldn’t know in which order the cups were placed. If we take the
simplest situation in which there are only two cups then the woman has 50% chance of
guessing correctly. If she did guess correctly we wouldn’t be that confident in concluding
that she can tell the difference between cups in which the milk was added first from those
in which it was added last, because even by guessing she would be correct half of the time.
However, what about if we complicated things by having six cups? There are 20 orders
in which these cups can be arranged and the woman would guess the correct order only
1 time in 20 (or 5% of the time). If she got the correct order we would be much more
50 DISCOVERING STATISTICS USING SPSS
before the tournament, but I can’t then change my mind
halfway through, or after the final game!
The situation in research is similar: we can choose any
hypothesis (rugby team) we like before the data are collected,
but we can’t change our minds halfway through
data collection (or after data collection). Likewise we
have to decide on our probability level (or betting odds)
before we collect data. If we do this, the process works.
However, researchers sometimes cheat. They don’t write
down their hypotheses before they conduct their experiments,
sometimes they change them when the data are
collected (like me changing my team after the World Cup
is over), or worse still decide on them after the data are
collected! With the exception of some complicated procedures
called post hoc tests, this is cheating. Similarly,
researchers can be guilty of choosing which significance
level to use after the data are collected and analysed, like
a betting shop changing the odds after the tournament.
Every time that you change your hypothesis or the
details of your analysis you appear to increase the chance
of finding a significant result, but in fact you are making
it more and more likely that you will publish results that
other researchers can’t reproduce (which is very embarrassing!).
If, however, you follow the rules carefully and
do your significance testing at the 5% level you at least
know that in the long run at most only 1 result out of every
20 will risk this public humiliation.
(With thanks to David Hitchin for this box, and with
apologies to him for turning it into a rugby example.)
The process I describe in this chapter works only if you
generate your hypotheses and decide on your criteria for
whether an effect is significant before collecting the data.
Imagine I wanted to place a bet on who would win the
Rugby World Cup. Being an Englishman, I might want
to bet on England to win the tournament. To do this I’d:
(1) place my bet, choosing my team (England) and odds
available at the betting shop (e.g. 6/4); (2) see which team
wins the tournament; (3) collect my winnings (if England
do the decent thing and actually win).
To keep everyone happy, this process needs to be
equitable: the betting shops set their odds such that
they’re not paying out too much money (which keeps
them happy), but so that they do pay out sometimes
(to keep the customers happy). The betting shop can
offer any odds before the tournament has ended, but it
can’t change them once the tournament is over (or the
last game has started). Similarly, I can choose any team
JANE SUPERBRAIN 2.4
Cheating in research 1
confident that she could genuinely tell the difference (and bow down in awe of her finely
tuned palette). If you’d like to know more about Fisher and his tea-tasting antics see David
Salsburg’s excellent book The lady tasting tea (Salsburg, 2002). For our purposes the takehome
point is that only when there was a very small probability that the woman could
complete the tea-task by luck alone would we conclude that she had genuine skill in detecting
whether milk was poured into a cup before or after the tea.
It’s no coincidence that I chose the example of six cups above (where the tea-taster had
a 5% chance of getting the task right by guessing), because Fisher suggested that 95% is
a useful threshold for confidence: only when we are 95% certain that a result is genuine
(i.e. not a chance finding) should we accept it as being true.8
The opposite way to look at
this is to say that if there is only a 5% chance (a probability of .05) of something occurring
by chance then we can accept that it is a genuine effect: we say it is a statistically significant
finding (see Jane Superbrain Box 2.5 to find out how the criterion of .05 became
popular!).
8
Of course, in reality, it might not be true – we’re just prepared to believe that it is.
51CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
Figure 2.9
Sir Ronald A.
Fisher, probably
the cleverest
person ever
(p < .0001)
textbook Statistical methods for research workers (Fisher,
1925)9
Fisher produced tables of these critical values, but
to save space produced tables for particular probability
values (.05, .02 and .01). The impact of this book should
not be underestimated (to get some idea of its influence
25 years after publication see Mather, 1951; Yates, 1951)
and these tables were very frequently used – even Neyman
and Pearson admitted the influence that these tables had
on them (Lehmann, 1993). This disastrous combination
of researchers confused about the Fisher and Neyman–
Pearson approaches and the availability of critical values
for only certain levels of probability led to a trend to report
test statistics as being significant at the now infamous
p < .05 and p < .01 (because critical values were readily
available at these probabilities).
However, Fisher acknowledged that the dogmatic
use of a fixed level of significance was silly: ‘no scientific
worker has a fixed level of significance at which from year
to year, and in all circumstances, he rejects hypotheses;
he rather gives his mind to each particular case in the
light of his evidence and his ideas’ (Fisher, 1956).
The use of effect sizes (section 2.6.4) strikes a balance
between using arbitrary cut-off points such as p < .05
and assessing whether an effect is meaningful within the
research context. The fact that we still worship at the shrine
of p < .05 and that research papers are more likely to be
published if they contain significant results does make
me wonder about a parallel universe where Fisher had
woken up in a p < .10 kind of mood. My filing cabinet full
of research with p just bigger than .05 are published and I
am Vice-Chancellor of my university (although, if this were
true, the parallel universe version of my university would be
in utter chaos, but it would have a campus full of cats).
This criterion of 95% confidence, or a .05 probability, forms
the basis of modern statistics and yet there is very little
justification for it. How it arose is a complicated mystery
to unravel. The significance testing that we use today is a
blend of Fisher’s idea of using the probability value p as an
index of the weight of evidence against a null hypothesis,
and Jerzy Neyman and Egron Pearson’s idea of testing
a null hypothesis against an alternative hypothesis. Fisher
objected to Neyman’s use of an alternative hypothesis
(among other things), and Neyman objected to Fisher’s
exact probability approach (Berger, 2003; Lehmann,
1993). The confusion arising from both parties’ hostility to
each other’s ideas led scientists to create a sort of bastard
child of both approaches.
This doesn’t answer the question of why we use .05.
Well, it probably comes down to the fact that back in the
days before computers, scientists had to compare their
test statistics against published tables of ‘critical values’
(they did not have SPSS to calculate exact probabilities for
them). These critical values had to be calculated by exceptionally
clever people like Fisher. In his incredibly influential
JANE SUPERBRAIN 2.5
Why do we use .05? 1
9
You can read this online at http://psychclassics.yorku.ca/Fisher/Methods/.
52 DISCOVERING STATISTICS USING SPSS
2.6.1.   Test statistics 1
We have seen that we can fit statistical models to data that represent the hypotheses that
we want to test. Also, we have discovered that we can use probability to see whether scores
are likely to have happened by chance (section 1.7.4). If we combine these two ideas then
we can test whether our statistical models (and therefore our hypotheses) are significant
fits of the data we collected. To do this we need to return to the concepts of systematic
and unsystematic variation that we encountered in section 1.6.2.2. Systematic variation is
variation that can be explained by the model that we’ve fitted to the data (and, therefore,
due to the hypothesis that we’re testing). Unsystematic variation is variation that cannot
be explained by the model that we’ve fitted. In other words, it is error, or variation not
attributable to the effect we’re investigating. The simplest way, therefore, to test whether
the model fits the data, or whether our hypothesis is a good explanation of the data we
have observed, is to compare the systematic variation against the unsystematic variation.
In doing so we compare how good the model/hypothesis is at explaining the data against
how bad it is (the error):
test statistic =
variance explained by the model
variance not explained by the model
=
effect
error
This ratio of systematic to unsystematic variance or effect to error is a test statistic, and
you’ll discover later in the book there are lots of them: t, F and χ2
to name only three. The
exact form of this equation changes depending on which test statistic you’re calculating,
but the important thing to remember is that they all, crudely speaking, represent the same
thing: the amount of variance explained by the model we’ve fitted to the data compared to
the variance that can’t be explained by the model (see Chapters 7 and 9 in particular for a
more detailed explanation). The reason why this ratio is so useful is intuitive really: if our
model is good then we’d expect it to be able to explain more variance than it can’t explain.
In this case, the test statistic will be greater than 1 (but not necessarily significant).
A test statistic is a statistic that has known properties; specifically we know how frequently
different values of this statistic occur. By knowing this, we can calculate the probability
of obtaining a particular value (just as we could estimate the probability of getting
a score of a certain size from a frequency distribution in section 1.7.4). This allows us to
establish how likely it would be that we would get a test statistic of a certain size if there
were no effect (i.e. the null hypothesis were true). Field and Hole (2003) use the ana­logy
of the age at which people die. Past data have told us the distribution of the age of death.
For example, we know that on average men die at about 75 years old, and that this distribution
is top heavy; that is, most people die above the age of about 50 and it’s fairly
unusual to die in your twenties. So, the frequencies of the age of demise at older ages are
very high but are lower at younger ages. From these data, it would be possible to calculate
the probability of someone dying at a certain age. If we randomly picked someone and
asked them their age, and it was 53, we could tell them how likely it is that they will die
before their next birthday (at which point they’d probably punch us). Also, if we met a
man of 110, we could calculate how probable it was that he would have lived that long
(it would be a very small probability because most people die before they reach that age).
The way we use test statistics is rather similar: we know their distributions and this allows
us, once we’ve calculated the test statistic, to discover the probability of having found a
value as big as we have. So, if we calculated a test statistic and its value was 110 (rather
like our old man) we can then calculate the probability of obtaining a value that large.
The more variation our model explains (compared to the variance it can’t explain), the
53CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
is that there is no effect in the population. All that a
non-significant result tells us is that the effect is not
big enough to be anything other than a chance finding
– it doesn’t tell us that the effect is zero. As Cohen
(1990) points out, a non-significant result should
never be interpreted (despite the fact that it often is)
as ‘no difference between means’ or ‘no relationship
between variables’. Cohen also points out that the
null hypothesis is never true because we know from
sampling distributions (see section 2.5.1) that two
random samples will have slightly different means,
and even though these differences can be very small
(e.g. one mean might be 10 and another might be
10.00001) they are nevertheless different. In fact,
even such a small difference would be deemed as
statistically significant if a big enough sample were
used. So, significance testing can never tell us that
the null hypothesis is true, because it never is.
	 Significant results: OK, we may not be able to accept
the null hypothesis as being true, but we can at least conclude
that it is false when our results are significant, right?
Wrong! A significant test statistic is based on probabilistic
reasoning, which severely limits what we can conclude.
Again, Cohen (1994), who was an incredibly lucid writer
on statistics, points out that formal reasoning relies on an
initial statement of fact followed by a statement about the
current state of affairs, and an inferred conclusion. This
syllogism illustrates what I mean:
¡	 If a man has no arms then he can’t play guitar:
¡	 This man plays guitar.
¡	 Therefore, this man has arms.
The syllogism starts with a statement of fact that
allows the end conclusion to be reached because you
can deny the man has no arms (the antecedent) by
	 The importance of an effect: We’ve seen already
that the basic idea behind hypothesis testing involves
us generating an experimental hypothesis and a null
hypothesis, fitting a statistical model to the data, and
assessing that model with a test statistic. If the probability
of obtaining the value of our test statistic by
chance is less than .05 then we generally accept the
experimental hypothesis as true: there is an effect in
the population. Normally we say ‘there is a significant
effect of …’. However, don’t be fooled by that word
‘significant’, because even if the probability of our
effect being a chance result is small (less than .05) it
doesn’t necessarily follow that the effect is important.
Very small and unimportant effects can turn out to be
statistically significant just because huge numbers of
people have been used in the experiment (see Field &
Hole, 2003: 74).
	 Non-significant results: Once you’ve calculated
your test statistic, you calculate the probability of
that test statistic occurring by chance; if this probability
is greater than .05 you reject your alternative
hypothesis. However, this does not mean that the null
hypothesis is true. Remember that the null hypothesis
JANE SUPERBRAIN 2.6
What we can and can’t conclude
from a significant test statistic 2
bigger the test statistic will be, and the more unlikely it is to occur by chance (like our 110
year old man). So, as test statistics get bigger, the probability of them occurring becomes
smaller. When this probability falls below .05 (Fisher’s criterion), we accept this as giving
us enough confidence to assume that the test statistic is as large as it is because our model
explains a sufficient amount of variation to reflect what’s genuinely happening in the
real world (the population). The test statistic is said to be significant (see Jane Superbrain
Box 2.6 for a discussion of what statistically significant actually means). Given that the
statistical model that we fit to the data reflects the hypothesis that we set out to test, then
a significant test statistic tells us that the model would be unlikely to fit this well if the
there was no effect in the population (i.e. the null hypothesis was true). Therefore, we can
reject our null hypothesis and gain confidence that the alternative hypothesis is true (but,
remember, we don’t accept it – see section 1.7.5).
54 DISCOVERING STATISTICS USING SPSS
2.6.2.   One- and two-tailed tests 1
We saw in section 1.7.5 that hypotheses can be directional (e.g. ‘the more someone reads
this book, the more they want to kill its author’) or non-directional (i.e. ‘reading more
of this book could increase or decrease the reader’s desire to kill its author’). A statistical
model that tests a directional hypothesis is called a one-tailed test, whereas one testing a
non-directional hypothesis is known as a two-tailed test.
Imagine we wanted to discover whether reading this book increased or decreased the desire
to kill me. We could do this either (experimentally) by taking two groups, one who had read this
book and one who hadn’t, or (correlationally) by measuring the amount of this book that had
been read and the corresponding desire to kill me. If we have no directional hypothesis
then there are three possibilities. (1) People who read this book want to kill me more
than those who don’t so the difference (the mean for those reading the book minus the
mean for non-readers) is positive. Correlationally, the more of the book you read, the
more you want to kill me – a positive relationship. (2) People who read this book want
to kill me less than those who don’t so the difference (the mean for those reading the
book minus the mean for non-readers) is negative. Correlationally, the more of the book
you read, the less you want to kill me – a negative relationship. (3) There is no difference
between readers and non-readers in their desire to kill me – the mean for readers
minus the mean for non-readers is exactly zero. Correlationally, there is no relationship
between reading this book and wanting to kill me. This final option is the null hypothesis.
The direction of the test statistic (i.e. whether it is positive or negative) depends on
whether the difference is positive or negative. Assuming there is a positive difference or
relationship (reading this book makes you want to kill me), then to detect this difference
we have to take account of the fact that the mean for readers is bigger than for non-readers (and
so derive a positive test statistic). However, if we’ve predicted incorrectly and actually reading
this book makes readers want to kill me less then the test statistic will actually be negative.
denying that he can’t play guitar (the consequent).10
A comparable version of the null hypothesis is:
¡	 If the null hypothesis is correct, then this test statistic
cannot occur:
¡	 This test statistic has occurred.
¡	 Therefore, the null hypothesis is false.
This is all very nice except that the null hypothesis is
not represented in this way because it is based on
probabilities. Instead it should be stated as follows:
¡	 If the null hypothesis is correct, then this test statistic
is highly unlikely:
¡	 This test statistic has occurred.
¡	 Therefore, the null hypothesis is highly unlikely.
If we go back to the guitar example we could get a
similar statement:
¡	 If a man plays guitar then he probably doesn’t play
for Fugazi (this is true because there are thousands
of people who play guitar but only two who play
guitar in the band Fugazi):
¡	 Guy Picciotto plays for Fugazi:
¡	 Therefore, Guy Picciotto probably doesn’t play
guitar.
This should hopefully seem completely ridiculous – the
conclusion is wrong because Guy Picciotto does play
guitar. This illustrates a common fallacy in hypothesis
testing. In fact significance testing allows us to say
very little about the null hypothesis.
10
Thanks to Philipp Sury for unearthing footage that disproves my point (http://www.parcival.org/2007/05/22/when-
syllogisms-fail/).
Why do you need
two tails?
55CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
What are the consequences of this? Well, if at the .05 level we needed to get a test statistic
bigger than say 10 and the one we get is actually –12, then we would reject the hypothesis
even though a difference does exist. To avoid this we can look at both ends (or tails) of the
distribution of possible test statistics. This means we will catch both positive and negative
test statistics. However, doing this has a price because to keep our criterion probability of
.05 we have to split this probability across the two tails: so we have .025 at the positive end
of the distribution and .025 at the negative end. Figure 2.10 shows this situation – the tinted
areas are the areas above the test statistic needed at a .025 level of significance. Combine the
probabilities (i.e. add the two tinted areas together) at both ends and we get .05, our criterion
value. Now if we have made a prediction, then we put all our eggs in one basket and look only
at one end of the distribution (either the positive or the negative end depending on the direction
of the prediction we make). So, in Figure 2.10, rather than having two small tinted areas
at either end of the distribution that show the significant values, we have a bigger area (the
lined area) at only one end of the distribution that shows significant values. Consequently, we
can just look for the value of the test statistic that would occur by chance with a probability
of .05. In Figure 2.10, the lined area is the area above the positive test statistic needed at a
.05 level of significance. Note on the graph that the value that begins the area for the .05 level
of significance (the lined area) is smaller than the value that begins the area for the .025 level
of significance (the tinted area). This means that if we make a specific prediction then we
need a smaller test statistic to find a significant result (because we are looking in only one tail
of the distribution), but if our prediction happens to be in the wrong direction then we’ll miss
out on detecting the effect that does exist. In this context it’s important to remember what I
said in Jane Superbrain Box 2.4: you can’t place a bet or change your bet when the tournament
is over. If you didn’t make a prediction of direction before you collected the data, you
are too late to predict the direction and claim the advantages of a one-tailed test.
2.6.3.   Type I and Type II errors 1
We have seen that we use test statistics to tell us about the true state of the world (to a
certain degree of confidence). Specifically, we’re trying to see whether there is an effect in
Figure 2.10
Diagram to show
the difference
between oneand
two-tailed
tests
56 DISCOVERING STATISTICS USING SPSS
our population. There are two possibilities in the real world: there is, in reality, an effect
in the population, or there is, in reality, no effect in the population. We have no way of
knowing which of these possibilities is true; however, we can look at test statistics and their
associated probability to tell us which of the two is more likely. Obviously, it is important
that we’re as accurate as possible, which is why Fisher originally said that we should be
very conservative and only believe that a result is genuine when we are 95% confident
that it is – or when there is only a 5% chance that the results could occur if there was not
an effect (the null hypothesis is true). However, even if we’re 95% confident there is still
a small chance that we get it wrong. In fact there are two mistakes we can make: a Type I
and a Type II error. A Type I error occurs when we believe that there is a genuine effect in
our population, when in fact there isn’t. If we use Fisher’s criterion then the probability of
this error is .05 (or 5%) when there is no effect in the population – this value is known as
the α-level. Assuming there is no effect in our population, if we replicated our data collection
100 times we could expect that on five occasions we would obtain a test statistic large
enough to make us think that there was a genuine effect in the population even though
there isn’t. The opposite is a Type II error, which occurs when we believe that there is no
effect in the population when, in reality, there is. This would occur when we obtain a small
test statistic (perhaps because there is a lot of natural variation between our samples). In
an ideal world, we want the probability of this error to be very small (if there is an effect
in the population then it’s important that we can detect it). Cohen (1992) suggests that the
maximum acceptable probability of a Type II error would be .2 (or 20%) – this is called the
β-level. That would mean that if we took 100 samples of data from a population in which
an effect exists, we would fail to detect that effect in 20 of those samples (so we’d miss 1
in 5 genuine effects).
There is obviously a trade-off between these two errors: if we lower the probability of
accepting an effect as genuine (i.e. make α smaller) then we increase the probability that
we’ll reject an effect that does genuinely exist (because we’ve been so strict about the level
at which we’ll accept that an effect is genuine). The exact relationship between the Type I
and Type II error is not straightforward because they are based on different assumptions:
to make a Type I error there has to be no effect in the population, whereas to make a Type
II error the opposite is true (there has to be an effect that we’ve missed). So, although we
know that as the probability of making a Type I error decreases, the probability of making
a Type II error increases, the exact nature of the relationship is usually left for the
researcher to make an educated guess (Howell, 2006, gives a great explanation of the
trade-off between errors).
2.6.4.    Effect sizes 2
The framework for testing whether effects are genuine that I’ve just presented has a few
problems, most of which have been briefly explained in Jane Superbrain Box 2.6. The
first problem we encountered was knowing how important an effect is: just because a test
statistic is significant doesn’t mean that the effect it measures is meaningful or important.
The solution to this criticism is to measure the size of the effect that we’re testing in a
standardized way. When we measure the size of an effect (be that an experimental manipulation
or the strength of a relationship between variables) it is known as an effect size.
An effect size is simply an objective and (usually) standardized measure of the magnitude
of observed effect. The fact that the measure is standardized just means that we can compare
effect sizes across different studies that have measured different variables, or have
used different scales of measurement (so an effect size based on speed in milliseconds
57CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
could be compared to an effect size based on heart rates). Such is the utility of effect
size estimates that the American Psychological Association is now recommending that all
psychologists report these effect sizes in the results of any published work. So, it’s a habit
well worth getting into.
Many measures of effect size have been proposed, the most common of
which are Cohen’s d, Pearson’s correlation coefficient r (Chapter 6) and the
odds ratio (Chapter 18). Many of you will be familiar with the correlation
coefficient as a measure of the strength of relationship between two variables
(see Chapter 6 if you’re not); however, it is also a very versatile measure of
the strength of an experimental effect. It’s a bit difficult to reconcile how the
humble correlation coefficient can also be used in this way; however, this is
only because students are typically taught about it within the context of nonexperimental
research. I don’t want to get into it now, but as you read through
Chapters 6, 9 and 10 it will (I hope) become clear what I mean. Personally,
I prefer Pearson’s correlation coefficient, r, as an effect size measure because it
is constrained to lie between 0 (no effect) and 1 (a perfect effect).11
However,
there are situations in which d may be favoured; for example, when group
sizes are very discrepant r can be quite biased compared to d (McGrath &
Meyer, 2006).
Effect sizes are useful because they provide an objective measure of the importance of
an effect. So, it doesn’t matter what effect you’re looking for, what variables have been
measured, or how those variables have been measured – we know that a correlation coefficient
of 0 means there is no effect, and a value of 1 means that there is a perfect effect.
Cohen (1988, 1992) has also made some widely used suggestions about what constitutes a
large or small effect:
rMM = .10 (small effect): In this case the effect explains 1% of the total variance.
rMM = .30 (medium effect): The effect accounts for 9% of the total variance.
rMM = .50 (large effect): The effect accounts for 25% of the variance.
It’s worth bearing in mind that r is not measured on a linear scale so an effect with r = .6
isn’t twice as big as one with r = .3. Although these guidelines can be a useful rule of thumb
to assess the importance of an effect (regardless of the significance of the test statistic), it
is worth remembering that these ‘canned’ effect sizes are no substitute for evaluating an
effect size within the context of the research domain that it is being used (Baguley, 2004;
Lenth, 2001).
A final thing to mention is that when we calculate effect sizes we calculate them for a
given sample. When we looked at means in a sample we saw that we used them to draw
inferences about the mean of the entire population (which is the value in which we’re
actually interested). The same is true of effect sizes: the size of the effect in the population
is the value in which we’re interested, but because we don’t have access to this value, we
use the effect size in the sample to estimate the likely size of the effect in the population.
We can also combine effect sizes from different studies researching the same question to
get better estimates of the population effect sizes. This is called meta-analysis – see Field
(2001, 2005b).
Can we measure how
important an effect is?
Can we measure how
important an effect is?
11
The correlation coefficient can also be negative (but not below –1), which is useful when we’re measuring a
relationship between two variables because the sign of r tells us about the direction of the relationship, but in
experimental research the sign of r merely reflects the way in which the experimenter coded their groups (see
Chapter 6).
58 DISCOVERING STATISTICS USING SPSS
2.6.5.    Statistical power 2
Effect sizes are an invaluable way to express the importance of a research finding. The
effect size in a population is intrinsically linked to three other statistical properties: (1) the
sample size on which the sample effect size is based; (2) the probability level at which we
will accept an effect as being statistically significant (the α-level); and (3) the ability of a test
to detect an effect of that size (known as the statistical power, not to be confused with statistical
powder, which is an illegal substance that makes you understand statistics better). As
such, once we know three of these properties, then we can always calculate the remaining
one. It will also depend on whether the test is a one- or two-tailed test (see section 2.6.2).
Typically, in psychology we use an α-level of .05 (see earlier) so we know this value already.
The power of a test is the probability that a given test will find an effect assuming that one
exists in the population. If you think back you might recall that we’ve already come across
the probability of failing to detect an effect when one genuinely exists (β, the probability
of a Type II error). It follows that the probability of detecting an effect if one exists must be
the opposite of the probability of not detecting that effect (i.e. 1 – β). I’ve also mentioned
that Cohen (1988, 1992) suggests that we would hope to have a .2 probability of failing
to detect a genuine effect, and so the corresponding level of power that he recommended
was 1 – .2, or .8. We should aim to achieve a power of .8, or an 80% chance of detecting
an effect if one genuinely exists. The effect size in the population can be estimated from
the effect size in the sample, and the sample size is determined by the experimenter anyway
so that value is easy to calculate. Now, there are two useful things we can do knowing that
these four variables are related:
1	 Calculate the power of a test: Given that we’ve conducted our experiment, we will
have already selected a value of α, we can estimate the effect size based on our
sample, and we will know how many participants we used. Therefore, we can use
these values to calculate β, the power of our test. If this value turns out to be .8 or
more we can be confident that we achieved sufficient power to detect any effects that
might have existed, but if the resulting value is less, then we might want to replicate
the experiment using more participants to increase the power.
2	 Calculate the sample size necessary to achieve a given level of power: Given that we
know the value of α and β, we can use past research to estimate the size of effect that
we would hope to detect in an experiment. Even if no one had previously done the
exact experiment that we intend to do, we can still estimate the likely effect size based
on similar experiments. We can use this estimated effect size to calculate how many
participants we would need to detect that effect (based on the values of α and β that
we’ve chosen).
The latter use is the more common: to determine how many participants should be
used to achieve the desired level of power. The actual computations are very cumbersome,
but fortunately there are now computer programs available that will do them
for you (one example is G*Power, which is free and can be downloaded from a link on
the companion website; another is nQuery Adviser but this has to be bought!). Also,
Cohen (1988) provides extensive tables for calculating the number of participants for a
given level of power (and vice versa). Based on Cohen (1992) we can use the following
guidelines: if we take the standard α-level of .05 and require the recommended power
of .8, then we need 783 participants to detect a small effect size (r = .1), 85 participants
to detect a medium effect size (r = .3) and 28 participants to detect a large effect
size (r = .5).
59CHAPTER 2 Everything you ever wanted to know about statistics (well, sort of)
What have I discovered about statistics? 1
OK, that has been your crash course in statistical theory – hopefully your brain is still
relatively intact. The key point I want you to understand is that when you carry out
research you’re trying to see whether some effect genuinely exists in your population
(the effect you’re interested in will depend on your research interests and your specific
predictions). You won’t be able to collect data from the entire population (unless you
want to spend your entire life, and probably several after-lives, collecting data) so you
use a sample instead. Using the data from this sample, you fit a statistical model to test
your predictions, or, put another way, detect the effect you’re looking for. Statistics boil
down to one simple idea: observed data can be predicted from some kind of model and
an error associated with that model. You use that model (and usually the error associated
with it) to calculate a test statistic. If that model can explain a lot of the variation in the
data collected (the probability of obtaining that test statistic is less than .05) then you
infer that the effect you’re looking for genuinely exists in the population. If the probability
of obtaining that test statistic is more than .05, then you conclude that the effect
was too small to be detected. Rather than rely on significance, you can also quantify the
effect in your sample in a standard way as an effect size and this can be helpful in gauging
the importance of that effect. We also discovered that I managed to get myself into
trouble at nursery school. It was soon time to move on to primary school and to new
and scary challenges. It was a bit like using SPSS for the first time.
Key terms that I’ve discovered
α-level
β-level
Central limit theorem
Confidence interval
Degrees of freedom
Deviance
Effect size
Fit
Linear model
Meta-analysis
One-tailed test
Population
Power
Sample
Sampling distribution
Sampling variation
Standard deviation
Standard error
Standard error of the mean (SE)
Sum of squared errors (SS)
Test statistic
Two-tailed test
Type I error
Type II error
Variance
Smart Alex’s stats quiz
1	 Why do we use samples? 1
2	 What is the mean and how do we tell if it’s representative of our data? 1
60 DISCOVERING STATISTICS USING SPSS
3	 What’s the difference between the standard deviation and the standard error? 1
4	 In Chapter 1 we used an example of the time taken for 21 heavy smokers to fall off
a treadmill at the fastest setting (18, 16, 18, 24, 23, 22, 22, 23, 26, 29, 32, 34, 34,
36, 36, 43, 42, 49, 46, 46, 57). Calculate the sums of squares, variance, standard
deviation, standard error and 95% confidence interval of these data. 1
5	 What do the sum of squares, variance and standard deviation represent? How do
they differ? 1
6	 What is a test statistic and what does it tell us? 1
7	 What are Type I and Type II errors? 1
8	 What is an effect size and how is it measured? 2
9	 What is statistical power? 2
Answers can be found on the companion website.
Further reading
Cohen, J. (1990). Things I have learned (so far). American Psychologist, 45(12), 1304–1312.
Cohen, J. (1994). The earth is round (p < .05). American Psychologist, 49(12), 997–1003. (A couple
of beautiful articles by the best modern writer of statistics that we’ve had.)
Field, A. P., & Hole, G. J. (2003). How to design and report experiments. London: Sage. (I am rather
biased, but I think this is a good overview of basic statistical theory.)
Miles, J. N. V., & Banyard, P. (2007). Understanding and using statistics in psychology: a practical
introduction. London: Sage. (A fantastic and amusing introduction to statistical theory.)
Wright, D. B., & London, K. (2009). First steps in statistics (2nd ed.). London: Sage. (This book has
very clear introductions to sampling, confidence intervals and other important statistical ideas.)
Interesting real research
Domjan, M., Blesbois, E., & Williams, J. (1998). The adaptive significance of sexual conditioning:
Pavlovian control of sperm release. Psychological Science, 9(5), 411–415.