to assess the effects of exercise on reducing cholesterol, a researcher selected a random sample of fifty people from a local gym who exercised regularly over the past year and another random sample of fifty people from the surrounding community who did not exercise regularly during the past year. they all reported to a clinic to have their cholesterol measured. the subjects were unaware of the purpose of the study, and the technician measuring the cholesterol was not aware of whether subjects exercised regularly. this is a(n)