The logic and computational details of two-sample
t-tests are described in Chapters 9-12 of the online text
Concepts & Applications of Inferential Statistics. For the independent-samples
t-test, this unit will perform both the "usual"
t-test, which assumes that the two samples have equal variances, and the alternative
t-test, which assumes that the two samples have
unequal variances.
(A good formulaic summary of the unequal-variances t-test can be found on the StatsDirect web site. A more thorough account appears in the online journal Behavioral Ecology.)