Sample Size Calculations and Statistical Power

Ana W. Capuano

Sample size determination and prospective power analysis are important factors in planning a statistical study, because a study executed with an inappropriate sample size may result in wasted resources. If the sample is too small, the inference goal may not be achieved and true effects may not be detected, and if it is too large, money and resources have been unnecessarily expended, and subjects may have been exposed unnecessarily to a drug or the treatment. Conversely, with an optimal sample, the investigator has increased chances of detecting true effects without wasting resources or exposing subjects to unnecessary risk. For this reason, many funding agencies required sample size determinations to be supported by statistical power analysis. To illustrate, the National Institutes of Health (NIH) Policy Manual (NIH, 1988) states that requests for approval from the Office of Management and Budget to conduct epidemiological studies should include a discussion of sample size ...

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