Introduction (Cont'd)

Sampling

While there are reasons to involve everyone in an evaluation (such as to avoid offending those not included), it can be both expensive and time-consuming with larger groups. So, it is usually necessary to select a sample of individuals to act as informants in an evaluation. The sample is the group of individuals chosen from the target community or group to participate in the evaluation. Samples can be chosen based on probability (random) or nonprobability methods:

Random Sample
In a random sample, everyone has an equal chance of being chosen. To have a random sample, you must set up a procedure for choosing participants that ensures that no one has a greater chance than anyone else of being chosen. See the Resources at the end of this chapter for links to more detailed information about probability sampling.

Nonprobability Sample
Individuals are not chosen randomly; some may have a greater chance of being chosen than others. A purposive or judgmental sample is a type of nonprobability sample in which the evaluator uses his or her knowledge of the population (for example, the program participants) and the evaluation's goals and objectives to choose individuals who are most appropriate. For example, to get different perspectives in focus groups, an evaluator might choose older adults who have the most experience with a program or, alternatively, are new to the program. You can also use purposive sampling to ensure that everyone is represented in the evaluation results.⁶

⁶ Rubin, A., and Babbie, E. (2001) Research Methods for Social Work (4th ed.). Belmont, CA: Wadsworth/Thomas Learning.