SPSS II – Solutions
1. To carry out a valid chi-square test, the ‘very poor’ and ‘poor’ diet categories
are combined. Prevalence of toxaemia is 29%, 10% and 7% respectively for
the diet categories ‘very poor or poor’, ‘fair’ and ‘good’ respectively. [chisquare(2)=13.1; p=0.001]. Hence the occurrence of toxaemia does appear to
be related to diet in pregnancy. If we take the ordering of the diet categories
into account the linear-by-linear association test gives p<0.001 indicating that
there is evidence of a linear trend in the decreased prevalence of toxaemia
with improved diet.
2. Of the normotensive patients, 3/90 died compared to 3/10 of the patients with
hypotension. This difference is statistically significant [Fisher’s Exact test;
p=0.013]. hence blood pressure does appear to be related to survival.
3. Of the controls, 54/317=17% used oestrogen compared with 152/317=48% of
the cases. Using McNemar’s test (for matched data), this difference is
significant, p<0.001
4. (i) AP selectin follows a log normal distribution, hence raw values were log
transformed prior to analysis. Geometric means are 154 and 247 respectively
for the placebo and aspirin groups. This difference is statistically significant
[two-sample t-test: t(28)=2.2; p=0.039]. The mean placebo/aspirin ratio is
0.63, 95% CI (0.40, 0.98) – this equates to a mean aspirin/placebo ratio of 1.59
(1.02, 2.50).
(ii) Haemoglobin is assumed to follow a Normal distribution. Mean values at
time 1 and time 4 are 145.2 and 146.3 respectively. This difference is not
statistically significant [paired t-test t(14)=1.4; p=0.18]. Mean difference = 1.07, 95% CI (-2.61, 0.57)
(iii) Age and haemoglobin are both assumed to follow a Normal distribution.
Pearson correlation = -0.44; p=0.016 over all 30 subjects. Hence there is a
significant negative association between age and haemoglobin.
5. SUD group: (n=10) median = 10, range (4,60)
Control group: (n=11), median=20, range (7,120)
No significant difference between the two groups [Mann-Whitney U-test;
p=0.097]
6. Blood glucose before: median=125, range (89, 158)
Blood glucose after: median = 136, range (96,212)
Significant difference between time points [ Wilcoxon signed rank sum test;
p=0.002].
SPSS2 – Solutions contd
7. Red cell folate follows a reasonable Normal distribution. The mean values are
316.6, 256.4 and 278.0 for groups I, II and II respectively. There are
significant differences between the groups (one-factor ANOVA, F(2,19)=3.71;
P=0.044). Using Scheffe’s multiple comparison test the difference between
groups I and II is identified to be statistically significant (p=0.045). Patients
receiving 50% oxygen mixture continuously for 24 hours have a significantly
higher red cell folate level compared with patients receiving 50% oxygen
mixture during the operation only.
8. Headache activity score does not follow a Normal distribution. The median
values are 74, 96 and –36 for the ‘relax+bio’, ‘relax’ and ‘untreated’ groups
respectively. The difference between the three groups is of borderline
significance (Kruskal-Wallis; p=0.057). The groups are small, hence
comparison of each pair of groups with Mann-Whitney U-tests is not very
powerful (and in fact all three P values are greater than 0.05 even without
allowing for multiple comparisons). However, it is reasonable to consider
whether the two actively treated groups together did better than the untreated
controls (Mann-Whitney U-test; p=0.032), supporting the suggestion that both
treatments are beneficial but that the study is too small to be able to
distinguish them.