Aspirin prevents blood from clotting and so aids prevent strokes. The Second European Stoke Study asked whether adding another anticlotting drug named dipyridamole would be more effective for the patients who had already had stroke. Here are the data upon strokes throughout the two years of the study:
Group Treatment Number of patients Number of strokes
1 Placebo 1649 250
2 Aspirin 1649 206
3 Dipyridamole 1654 211
4 Both 1650 157
Does the data provide convincing evidence of difference in the effectiveness of four treatments? Perform an appropriate test at the ?=0.05 significance level. Show your work