example: To create a more efficient and profitable business, an HMO has decided that doctors may only prescribe one drug for a particular condition, even if there are other similar drugs on the market. Two competing pharmaceutical companies have drugs that are designed to reduce cholesterol levels. Both drugs have been on the market for several years and are effective for most people.
To obtain subjects for the study, a list of clinics run by the HMO was obtained and three clinics were randomly selected. Within each clinic, people who had been diagnosed with border-line high to high levels of total cholesterol were then classified based on their gender, age, race, and their initial cholesterol count (borderline or high), a stratified random sample was then taken from each of the clinics.