Power Analysis

Power analysis is often used when designing a study to determine an appropriate sample size. Somewhat controversially, power analyses can also be conducted after the data have been collected in order to determine whether the design yielded sufficient power to detect an effect that truly exists in the underlying population; for more information on this controversy check out the following article:
Hoenig, J. M., & Heisey, D. M. (2001). The abuse of power: The pervasive fallacy of power calculations for data analysis. The American Statistician, 55(1), 19-24.