Directional decisions for two-tailed tests: Power, error rates, and sample size.

One imposing directional decisions on nondirectional tests will overestimate power, underestimate sample size, and ignore the risk of Type III error (getting the direction wrong) if traditional calculations—those applying to nondirectional decisions—are used. Usually trivial with the z test, the errors might be important where α is large and effect size is small or with tests using other distributions. One can avoid the errors by using calculations that apply to directional decisions or by using a directional two-tailed test at the outset, a conceptually simpler solution. With a revised concept of power, this article shows calculations for the test; explains how to find its power, Type III error risk, and sample size in statistical tables for traditional tests; compares it to conventional one- and two-tailed tests and to one- and two-sided confidence intervals; and concludes that when a significance test is planned it is the best choice for most purposes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)