information:extraction
Two-sample t -tests

A two-sample location test of the null hypothesis such that the means of two populations are equal. All such tests are usually called Student's t -tests, though strictly speaking that name should only be used if the variances of the two populations are also assumed to be equal; the form of the test used when this assumption is dropped is sometimes called Welch's t -test. These tests are often referred to as unpaired or independent samples t -tests, as they are typically applied when the statistical units underlying the two samples being compared are non-overlapping.

Two-sample t -tests for a difference in means involve independent samples (unpaired samples) or paired samples. Paired t -tests are a form of blocking, and have greater power (probability of avoiding a type II error, also known as a false negative) than unpaired tests when the paired units are similar with respect to "noise factors" (see confounder) that are independent of membership in the two groups being compared. In a different context, paired t -tests can be used to reduce the effects of confounding factors in an observational study.