What is a retrospective power and sample size analysis?

A retrospective power and sample size analysis is an analysis that you use to determine whether a test that you already performed had enough power.

Example

An engineer performs a hypothesis test, but the test does not detect the difference that the engineer specified. The engineer then performs a retrospective power and sample size analysis, which indicates that the power to detect this difference is low. The engineer might want to increase power and then perform the test again.