Question: In a famous study of the preventive benefits for healthy people of low- dose aspirin a total of 22,071 healthy individuals were randomly assigned to two groups; one group received the treatment and the other a placebo. The table below shows the subjects classified by whether or not they had had a heart attack in the five years of the study. The table also shows the numbers of strokes (a possible side effect) in the two groups.

1. A ‘chi—square contingency table test’ was carried out and gave 28.8 as the value of the test statistic. Explain how the statistic was calculated and the rationale for the calculations. What hypothesis is tested by this statistic? What is the critical value for this test, with significance level of 0.05? Interpret the result of the test.
2. Test the statistical significance of the difference between the proportions of heart attacks in the two groups. What hypothesis is tested by the significance test?
3. Calculate a 95% confidence interval based on the difference between the two sample proportions for strokes. What are the practical implications of this interval?
4. Calculate a 95% confidence interval based on the proportion of heart attacks in the placebo group. Explain the meaning of this interval.

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