An epidemiologist conducted a prospective cohort study of exercise and stroke, with follow-up from 1975 to 2010. She defined light exercise as exercising, on average, 30 minutes or less per day, moderate exercise as exercising, on average, more than 30 minutes to 1 hour per day, and heavy exercise as exercising, on average, more than 1 hour per day. From 1975 through 1979, subject Q exercised 45 minutes per day. From 1980 through 1989, subject Q exercised 1.5 hours per day. From 1990 through 2004, subject Q exercised 25 minutes per day. Subject Q had a stroke in 2004. Assuming no induction period, which exposure category would subject Q be assigned to when she experienced the study outcome?

Question

BlackTom AI · Accepted Answer

The task asks which exposure category subject Q would be assigned to at the time she experienced the stroke in 2004, given the exercise history and no induction period.

Option a. Light exercise
From 1990 through 2004, subject Q exercised 25 minutes per day, which fits the definition of light exercise (30 minutes or less per day on average). Since the outcome (stroke) occurs in 2004, the exposure status at the time of the event would be based on the most recent measurement, i.e., 1990–2004. Therefore, the appropriate exposure category at the time of outcome is light exercise.

Option b. Moderate exercise
Moderate exercise is defined as more than 30 minutes up to 1 hour per day. This category applies to a period when Q exercised 45 minutes per day (1975–1979). However, the question concerns the exposure status at the time of outcome in 2004, and the most recent exposure before the event is 25 minutes per day (light). Consequently, moderate exercise would not describe her exposure at the time of the stroke.

Option c. Heavy exercise
Heavy exercise is more than 1 hour per day. This category would apply to the 1980–1989 period when Q exercised 1.5 hours per day. But by 2004, with 25 minutes per day, heavy exercise would no longer characterize her exposure at the time of the outcome.

In summary, even though earlier intervals show other categories (moderate in 1975–1979 and heavy in 1980–1989), the exposure status at the time the stroke occurred (2004) corresponds to the most recent exposure, which is light exercise (≤30 minutes per day).

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