Estimation of Type III Error and Power for Directional Two-Tailed Tests Using PROC POWER

By imposing directional decisions on traditional two-tailed tests, you will overestimate power, underestimate sample size, and ignore the risk of Type III error. You can avoid these problems by using the directional two-tailed test. This paper demonstrates how to use PROC POWER to estimate the prospective power and Type III error for one-sample, two-sample and paired t tests for means, and binomial tests for proportions, using the directional two-tailed testing procedure. Examples are given with SAS ® codes accompanied by selected results. Power and Type III error for the directional two-tailed sign test with unknown direction is also discussed and illustrated with a SAS macro written by the author.