4.51 Cholesterol level seems to be an important factor in myocardial infarctions (M.I.). In Table 4.30 the data below give the number of M.I.’s over the number in the cholesterol group for each arm of the study. In this study, patients were randomly assigned to the aspirin or placebo group.

a Did the randomization in the study seem to do a good job of balancing the cholesterol levels between the two groups? Explain.
b Construct a 2 2 table of aspirin or placebo versus M.I. response for each of the four cholesterol levels. Reduce the data in each table to the odds ratio.
c Compare the four odds ratios found in part (b).
Comment on the relationship between the effect of aspirin on heart attacks and the cholesterol levels. Do you see why odds ratios are handy tools for summarizing data in a 2 2 table?

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Cholesterol Levels by Study Groups Cholesterol Level Aspirin Placebo (mg per 100 ml) Groups Groups 159 2/382 9/406 160-209 12/1,587 37/1,511 210-259 26/1,435 43/1,444 260 14/582 23/570