We all know that exercise is good for us. But can exercise prevent or delay the symptoms 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 by selecting all patients that met certain health criteria. During the course of the study, the disease developed in 387 people. The study found that men who had participated 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?
a. convenience
b. volunteer
c. judgment
d. simple random
e. cluster