Health Study on Aspirin

US physicians, health study consisted of two randomized clinical trails in one. the first tested the hypothesis that 345 milligrams of aspirin taken every other day reduces mortality from cardiovascular disease. The second tested whether 50 mg B-carotene taken on alternate days decreases the incidence of cancer. 261,248 male physicians between the ages of 40 and 84 were invited to participate in the trail. Of those who responded, 59,285 were willing to participate. After the exclusion of those physicians who had a history of medical disorders, or who were currently taking aspirin or had negative reaction to aspirin. 22,071 physicians were randomized into one of the four treatment groups; (1) buffered aspirin and B-carotene, (2) buffered aspirin and a B-carotene placebo, (3) aspirin placebo and B- carotene, and (4) aspirin placebo and B-carotene placebo. Thus half were assigned to receive aspirin and half to receive B-carotene.

The study was conducted as double bind study, in which neither the participants nor the investigators responsible for following the participants knew to which group a participant belonged. The result of the study concerning myocardial infections are given in the table;

American Study
Aspirin (n = 11,037) Placebo (n = 11,034)
Myocardial infection
Fatal 5 18
Nonfatal 99 171
Total 104 189

The other objective of the British study was to determine whether 500 milligram of aspirin taken daily would reduce the incidence of and mortality from cardiovascular disease. After exclusions, 5139 doctors were were randomly allocated to take the aspirin, unless some problem developed, and one-third were randomly allocated to avoid aspirin. Placebo tablets were not used, so the study was not blind! The results of the British study:

British Study
Aspirin (n = 3429) Control (n = 1710)
Myocardial Infection
Fatal 89 (47.3) 47 (49.6)
Nonfatal 80 (42.5) 41 (43.3)
Total 169 (89.8) 88 (92.9)

To account for unequal sample sizes, the British study reported rates per 10,000 subject-years alive (given in parentheses).

a. Test whether American study does in fact indicate that the rates of heart attacks for the physicians taking 325 mg of aspirin every other day is significantly different from the rate those on the placebo. Is the claim justified?
b. Repeat the analysis using the Brithish study in which one group took 500 mg of aspirin every day and the control group took none. Based on their data, is the British claim justified?
c. Can you think of some possible reasons the results of the two which are alike in some respects, produced such different conclusions?