A market research firm has been asked to survey the population of people in a particular city who are 16 years of age or over with respect to their preferences for television programming. To do this the firm divides the list of the target population into five age groups: 16 to 25, 26 to 35, 36 to 45, 46 to 55, and 56 or older. From each of these age groups a simple random sample of 100 people is selected for a total sample of size 500 individuals. Assume that all of these selected individuals respond honestly to the survey questions. The resulting sample is an example of what kind of sample design?