Figure 2From: To test or not to test: Preliminary assessment of normality when comparing two independent samples Estimated Type I error probability of the two-sample t test at α = .05 after the residuals had passed the Shapiro-Wilk test for normality at α pre = .100, .050, .010, .005 (conditional), and without pretest (unconditional). Samples of equal size from the (A) exponential, (B) uniform, and (C) normal distribution.Back to article page