Fundamentals of Statistics contains material of various lectures and courses of H. Lohninger on statistics, data analysis and here for more.

Two-Sample t-Test

So far we have tested sample means to ascertain how well they agree with μ of the underlying population. However, we often want to compare the mean of different populations, e.g. compare products of two different companies, or one medical therapy vs. another, etc. In effect, we use the same procedures as when inferring from a population. Modifications have to be made to accommodate the fact that we are comparing two estimated values and not an estimated with a true value.