When should I choose Welch’s t-test over a standard Student’s t-test for group comparisons?

All answers to this question.

You should definitely go with Welch’s t-test. The standard Student’s t-test assumes "homoscedasticity," meaning the variances of the two groups are roughly equal. When that assumption is violated—especially with unequal sample sizes—the pooled variance estimate becomes biased, leading to an unreliable p-value. Welch’s t-test is much more robust because it doesn't pool the variances; instead, it uses the Satterthwaite-Welch equation to adjust the degrees of freedom. In a 2023 A/B test analysis for a retail client, using Student's t-test gave us a false significant result, whereas Welch’s correctly identified that the difference was within the margin of error once the variance was accounted for.

   Answered 2025-05-15 by Amanda Richardson

Does Welch's t-test lose a significant amount of statistical power compared to the standard version if the variances actually turn out to be equal?

   Answered 2025-05-20 by Thomas Brown

• Thomas, the power loss is negligible, often less than 1-2%. In fact, many modern statisticians argue that Welch’s t-test should be the default choice because it performs just as well as Student’s when variances are equal and far outperforms it when they are not. It's much safer to use Welch’s by default than to risk a high Type I error rate.

     Commented 2025-05-25 by David Miller

I always run Levene’s test first to check for equality of variances. If it fails, I switch to Welch’s immediately to stay on the safe side.

   Answered 2025-06-05 by Patricia Taylor

• That’s a standard workflow, Patricia. However, keep in mind that testing for assumptions can sometimes be problematic itself; Welch’s is so robust that many just skip the preliminary test and use it directly.

     Commented 2025-06-10 by Jennifer Miller