Aspirin prevents blood from clotting and so helps prevent strokes. The Second European Stroke Prevention Study asked whether adding another anticlotting drug named dipyridamole would be more effective for patients who had already had a stroke. Here are the data on strokes during the two years of the study:

a. Summarize these data in a two-way table.

b. Do the data provide convincing evidence of a difference in the effectiveness of the four treatments at the α = 0.05 significance level?

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Group Treatment Number of patients Number who had a stroke 1 Placebo 1649 250 2 Aspirin 1649 206 3 Dipyridamole 1654 211 4 Both 1650 157