The purpose of this article is to present the concepts involved in selecting a minimum sample size of subjects to represent a larger population. In determining sample size, it is important that the sample studied adequately represents the population to which the researcher is generalizing. The size of the study should be considered early in the planning phase of a research study. Often no formal sample size is ever calculated. Instead, the number of subjects available to the investigators during some period of time determines the size of the study. Many clinical trials that do not carefully consider sample size requirements lack the power and ability to detect intervention effects of fairly substantial magnitude and clinical importance.
Since the cost of a study is partially dependent on the number of subjects sampled, it is important to determine the fewest number of subjects required to yield valid results. Therefore, two key elements in a research design are the methodology used to select a sample and the minimum number of subjects chosen.
Three methods of determining the minimum sample size are presented. The first method is based on the requirement of a specific level of significance. The second method involves power and effect-size concepts. The third method is based on a required percent change due to the treatments.
The inherent limitations of these techniques are reviewed, and additional reading is suggested.
© 1995 American Academy of Orthotists & Prosthetists