Nonprobability sampling is used by researchers to find members of a population pool when this population cannot be enumerated to allow for a probability sample to be constructed. There are four general categories of nonprobability sampling: convenience, quota, purposive, and snowball sampling.

A convenience sample is a selected group from a particular population that is chosen based on their accessibility to the researcher. Examples of convenience samples include selecting neighborhood residents by inviting participants from those entering a supermarket, selecting grade school students from the local elementary school, and selecting undergraduate students from a freshman-level class.

A quota sample is a convenience sample in which the researcher seeks to control some variability in his or her sample. As in a probability sampling—stratified sampling method—a researcher may preselect ...