Does heavy exercise increase the risk of myocardial infarction? Mittleman et al. (1993)...

Does heavy exercise increase the risk of myocardial infarction? Mittleman et al. (1993) studied this question by examining the activities of 1228 patients who had suffered myocardial infarctions. It was determined whether each patient had participated in heavy exertion in the hour before the onset of the infarction and also whether each had participated in heavy exertion at the same time the previous day. Their results are displayed in the following table:

Does the study demonstrate that heavy exertion is associated with myocardial infarction? How does the design of this study relate to that of the cell phone study in Example B of Section 13.5?