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?

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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?

   D. During an outbreak of flu, 80  of 170 students of faculty of  teaching,
   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?

   E. The incidence of asthma in the adult population of Czech Republic 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?