The one-sample t test compares the mean of a sample to a given value. The two-sample t test compares the means of two samples. The paired t test compares the mean of the differences in the observations to a given number. PSS provides power and sample size computations for all of these types of t tests. For more information about power and sample size analysis for t tests, see Chapter 89: The POWER Procedure.
The two-sample t test tests for differences or ratios between means for two groups. The groups are assumed to be independent. This example describes three examples using the two-sample t test: for equal variances, for unequal variances, and for mean ratios.