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**University of York
Department of Health Sciences
**

**Applied Biostatistics**

**Suggested answers to exercise: The analysis of cross-tabulations
Question 1
**

a) *What is meant by ‘chi-squared = 23.98, P<0.001?’ *This is the result of the chi-squared
test which tests the null hypothesis that there is no association between *P. alcalifaciens*
and foreign travel. The value 23.98 is the test statistic which will follow a chi-squared
distribution with 1 degree of freedom if the null hypothesis is true. P<0.001 tells us that
the probability of these data or more extreme data occurring if the null hypothesis were
true is smaller than 0.001 and so we have good evidence that the null hypothesis is not
true and conclude that an association exists.

b) *What conditions do the data have to meet for the test to be valid? *The chi-squared test is
a large sample test and the usual rule is that the large sample approximation holds if all
expected frequencies are greater than 5 for a 2 by 2 table. Although one observed
frequency is 5, no expected values will be as small. This is because if the null hypothesis
were true then the overall probability of being positive for *P. alcalifaciens* would be
28/627 = 0.04 and this proportion would apply to those who have and those who have
not travelled abroad. Thus the expected numbers positive for *P. alcalifaciens* would be
254 × 28/627 = 11.3 for those who have travelled abroad and 373 × 28/627 = 16.7 among
those who have not travelled abroad. The other expected values can be calculated in a
similar way but will be large because the expected values must add to the marginal totals
for each row and column.

c) *What conclusions can be drawn from these data? *The study shows that there is a
statistically significant association between travelling abroad and being positive for *P.
alcalifaciens* among people with gastroenteritis. We cannot conclude from this that *P.
alcalifaciens* was the cause of the gastroenteritis. We can only conclude that an
association between *Providencia* and foreign travel exists.

d) *What other information would be useful in deciding whether P. alcalifaciens was a likely
cause of gastroenteritis in travellers? *We need a control group. We could look at the
number of positive screens for *P. alcalifaciens* among subjects without diarrhoea cross-
classified according to whether or not they had recently travelled abroad. This would tell
us if the observed association between travel and *P. alcalifaciens* was a general one or
one specific to those with diarrhoea.

**Question 2
**

a) *What is wrong with this statement and what analysis should they have done? *The
authors appear to have tested each line of the three by two contingency table separately.
This would involve doing three significance tests using the same data. This increases the
chance of a type I error, a significant difference where there is none in the population.
The authors could have done a chi-squared test for a three by two contingency table.

**Question 3
**

a) *What method could we use to test the null hypothesis that the two classifications are
related, and why?* The expected values here must all be small, as all four of them must
sum to 8. Hence we cannot use a chi-squared tests, but must use Fisher’s exact test.
This is not significant, P = 0.067. The invalid chi-squared test would give
chi-squared = 6.00, df = 1, P = 0.014.

**Question 4
**

a) *What is a trend test and how would you interpret the one presented here? *The trend test
is the Armitage chi-squared test for trend or the Mantel-Haenszel test for trend. It works
by assigning numerical values to each category and then estimating the best prediction of
one variable by the other as a simple *y* = constant1 + constant2×*x*. The chi-squared test
for trend tests the null hypothesis that there is no such prediction and constant2 = 0. The
calculated chi-squared value is 20.6 with 1 degree of freedom with an associated P-value
smaller than 0.001. This provides strong evidence for a trend in the proportions and so
we would conclude that the proportion of males who are seropositive increases with age.

b) *What would be the advantages and disadvantages compared to a chi-squared test for
association in a contingency table? *The test for trend takes into account the ordering of
the age groups, which the ordinary contingency table chi-squared does not. Hence the
test for trend has much greater power to detect a steady increase (or decrease) in
seropositivity with age. However the test has less power to detect non-linear
relationships, such as seropositivity being higher among young men and older men than
among those in the middle of the age range. Such a relationship would produce a non-
significant trend test.

c) *Suggest an alternative way of testing the difference in age in the two seropositivity
groups, assuming that the raw data were available.* Assuming that age was recorded in
years and given the large samples (176 seropositive, 305 seronegative), we could
compare the difference in mean age between the two groups using the large sample
Normal comparison method (z test).

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