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.