The "t"-test provides an exact test for the equality of the means of two i.i.d. normal populations with unknown, but equal, variances. (Welch's "t"-test is a nearly exact test for the case where the data are normal but the variances may differ.) For moderately large samples and a one tailed test, the "t"-test is relatively robust to moderate violations of the normality assumption. In large enough samples, the t-test asymptotically approaches the "z"-test, and becomes robust even to large deviations from normality.