Practicals 6 – Contingency tables
1. 128 Plants were cultivated in a green-house experiment. In this experiment, half of the pots were
fertilized by potassium while the other half not. Flowering was recorded after two weeks of the
continuous potassium treatment. 51 plants of potassium treated produced a flower while in the control
plant it was 33. Does potassium significantly affect probability of flowering?
2. 812 citizens of the same age were monitored during their lifetime in a town in Central China by a local
medical authorities. It was recorded whether they drink tea and if so, whether they prefer green or
black tea. It was also monitored who of the experimental group developed cancer by 70 years of age.
The data were as following:
healthy cancer
no tea 130 55
green tea 357 65
black tea 160 45
Do the data suggest that drinking tea could protect against cancer? If so, is there any difference between
the types of tea?
3. In a total set of 500 vegetation plots, 150 were of the wet meadow habitat.
Carex nigra had 30 records in the dataset. Of that 26 was in wet meadows.
Festuca rubra had 246 records. Of that 120 was in wet meadows.
Cannabis sativa had 10 records in the dataset. Of that none was in wet meadows.
How are these species associated with the wet meadow habitat?
4. During a month period, 115 persons were taken to hospital in Sierra Leone due to cholera. It was also
recorded whether they had been vaccinated against tetanus. 55 patients survived cholera of 60
vaccinated against tetanus while 15 patients survived of 55 non-vaccinated. Does vaccination against
tetanus protect also against cholera?
5. During an outbreak of flu, 80 of 170 students of faculty of education, 190 of 220 of students of faculty
of science and 22 of 290 of students of faculty of philosophy became ill. Does susceptibility to flu differ
among students of the faculties?
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A. 10% of 120 female students of faculty of teaching became pregnant during a certain period. The
conception rate accounted for 20% of 160 of students of faculty of science and 5% of 160 at faculty of
philosophy. Is there a difference in probability of becoming pregnant among female students of the
three individual faculties? If so, which faculty differs from which?
B. A dentist has 350 patients in his database that undergo regular checks for the teeth health. In
addition to the information on tooth decay, he asks his patients whether they drink tea unsweetened,
sweetened by sugar or sweetened by honey. The resulting data are following:
Sugar: 60 with decay/18 without decay
Honey: 90 with decay/45 without decay
Unsweetened: 50 with decay/87 without decay
Does the type of tea sweetening affect health of teeth? If so, how?
C. Two species Humulus lupullus and Alnus incana have following records in a set of vegetation plots:
cooccurrence – 26 plots, Humulus only 6 plots, Alnus only 81 plots, none of the species 23 plots. Are
these species significantly associated? If yes, how?
D. A group of student volunteers agreed to take part in an experiment studying the effect of supportive
drugs on cognitive capacity. Before taking an exam, the students were randomly assigned to four groups.
Members of each group were administered a type of drug potentially supporting their performance at
the exam. The exam results are summarized in the table below
passed failed
Tea 65 37
Coffee 60 41
Chocolate 71 29
energy drink 55 46
Do the drugs significantly support students performance at the exam? If so, can the identified trend be
interpreted as really causal effects? In case of significant result, describe the differences between the
effects of individual drug types.
E. The incidence of asthma in the adult population of Czech Republic (8,700,000 people in total) is 6%.
Two large Czech cities were surveyed for asthma incidence based on a sample of 500 randomly chosen
adult inhabitants in each of the cities. 26 persons within the sample were diagnosed to suffer from
asthma in Brno while the figure was 106 persons in Ostrava. Does asthma incidence in these cities
differ significantly from the nation-wide incidence?