We all know that exercise is good for us. But can exercise prevent or delay the symptons of Parkinson's disease? A recent study by the Harvard School of Public Health Studied 48,000 men and 77,000 women who were relatively healthy and middle-aged or older. During the course of the study, the disease developed in 387 people. The study found that men who had participation in some vigorous activity at least twice a week in high school, college and up to age 40 had a 60% reduced risk of Parkinson's disease developing. The study found no such reduction for women. What type of sampling does this represent?