Why Psychologists Should by Default Use Welch’s <i>t</i>-test Instead of Student’s <i>t</i>-test
DOI:
https://doi.org/10.5334/irsp.82Keywords:
Welch’s t-test, Student’s t-test, homogeneity of variance, Levene’s test, Homoscedasticity, statistical power, type 1 error, type 2 errorAbstract
When comparing two independent groups, psychology researchers commonly use Student’s t-tests. Assumptions of normality and homogeneity of variance underlie this test. More often than not, when these conditions are not met, Student’s t-test can be severely biased and lead to invalid statistical inferences. Moreover, we argue that the assumption of equal variances will seldom hold in psychological research, and choosing between Student’s t-test and Welch’s t-test based on the outcomes of a test of the equality of variances often fails to provide an appropriate answer. We show that the Welch’s t-test provides a better control of Type 1 error rates when the assumption of homogeneity of variance is not met, and it loses little robustness compared to Student’s t-test when the assumptions are met. We argue that Welch’s t-test should be used as a default strategy.
Publisher’s Note: A correction article relating to this paper has been published and can be found at https://www.rips-irsp.com/articles/10.5334/irsp.661/.
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