What is a power analysis?

A power analysis is a calculation that helps you determine a minimum sample size for your study. It’s made up of four main components. If you know or have estimates for any three of these, you can calculate the fourth component.

  • Statistical power: the likelihood that a test will detect an effect of a certain size if there is one, usually set at 80% or higher.
  • Sample size: the minimum number of observations needed to observe an effect of a certain size with a given power level.
  • Significance level (alpha): the maximum risk of rejecting a true null hypothesis that you are willing to take, usually set at 5%.
  • Expected effect size: a standardized way of expressing the magnitude of the expected result of your study, usually based on similar studies or a pilot study.