Next section: Estimating power

Several other factors affect power. One factor is whether the population distribution is normal. Deviations from the assumption of normality usually lower power. A second factor affecting power is the type of statistical procedure used. Some of the distribution-free tests are less powerful than other tests when the distribution is normal distributions but more powerful when the distribution is highly-skewed. One-tailed tests are more powerful than two-tailed tests as long as the effect is in the expected direction. Otherwise, their power is zero.

The choice of an experimental design can have a profound effect on power. Within-subject designs are usual much more powerful than between-subject designs.

Next section: Estimating power