To test or not to test: Preliminary assessment of normality when comparing two independent samples.

ABSTRACT:

Background

Student's two-sample t test is generally used for comparing the means of two independent samples, for example, two treatment arms. Under the null hypothesis, the t test assumes that the two samples arise from the same normally distributed population with unknown variance. Adequate control of the Type I error requires that the normality assumption holds, which is often examined by means of a preliminary Shapiro-Wilk test. The following two-stage procedure is widely accepted: If the preliminary test for normality is not significant, the t test is used; if the preliminary test rejects the null hypothesis of normality, a nonparametric test is applied in the main analysis.

Methods

Equally sized samples were drawn from exponential, uniform, and normal distributions. The

SUBMITTER: Rochon J 

PROVIDER: S-EPMC3444333 | biostudies-literature | 2012 Jun

REPOSITORIES: biostudies-literature