Researchers are conducting an experiment to determine the effects of a particular collagen supplement on joint health in adults. They randomly select 1,200 patients with no prior history of joint pain from a national hospital database in these age groups: 21 to 40 years, 41 to 60 years, and over 60 years. In each group, half the subjects receive the collagen supplement and the other half receive a placebo, although none of the subjects know which they receive.

To create randomization, this study uses ___ to reduce variability from known factors. The study also ___ a control group and applies ___ sampling methods.