The cost of studying an entire population to answer a specific question is usually prohibitive in terms of time, money and resources. Therefore, a subset of subjects representative of a given population must be selected; this is called sampling. The concepts involved in selecting subjects to represent the larger population are presented. Sampling errors and associated determining factors are reviewed.
Definitions of the research populations, including target and accessible groups, are given. The inclusion and exclusion criteria required to refine the accessible population to a re-searchable subgroup are explained, and an example is provided. The two types of sampling methods, probability and nonprobability, are defined and presented with their respective types. Probability sampling includes simple random sampling, systematic sampling, stratified sampling, cluster sampling and disproportional sampling. Nonprobability sampling includes convenience sampling, consecutive sampling, judgmental sampling, quota sampling and snowball sampling.
The goals and concepts related to recruitment are reviewed with application to survey and experimental research. Three steps are suggested for obtaining an appropriate research sample: (1) clearly define the target population, (2) define the accessible population, and (3) define the steps and effort that will be employed to recruit subjects for study.