Approximate Power for Studies Using the t Test for Independent Means Testing Hypotheses at the 0.05 Significance Level Number of Participants in Each Group Effect Size Small (0.20) Medium (0.50) Large (0.80) One-tailed test 10 0.11 0.29 0.53 20 0.15 0.46 0.80 30 0.19 0.61 0.92 40 0.22 0.72 0.97 50 0.26 0.80 0.99 100 0.41 0.97 * Two-tailed test 10 0.07 0.18 0.39 20 0.09 0.33 0.69 30 0.12 0.47 0.86 40 0.14 0.60 0.94 50 0.17 0.70 0.98 100 0.29 0.94 * *Nearly 1