To answer most evaluation questions, data will be gathered from the entire population because the population of interest is relatively small, and external validity—or generalizability—beyond the group of interest is not a priority

To answer most evaluation questions, data will be gathered from the entire population because the population of interest is relatively small, and external validity—or generalizability—beyond the group of interest is not a priority. Methods of random sampling can be used when the group is large and generalizability is important. Purposive sampling is useful when the evaluation can benefit from information from a particular, identified subgroup to learn more about that group. Purposive sampling is often used in conducting intensive interviews and in case studies.