Aresearcher recruits 2000 participants for a study. they are assigned randomly to control and treatment groups by an independent administrator, so the researcher does not know who was assigned to which group. the treatment group subjects are instructed to take a dietary supplement each day for 30 days. the control group subjects are told to follow their normal diet with no differences. all subjects have their blood pressure measured by an independent technician (not the researcher) at the start and at the end of the 30 day period.