In many applications we have two or several groups of observations rather than a single set, and the goal of the analysis is to compare the distributions of the groups. For instance, we can again consider the gross national products of all countries in the United Nations in 1980, but separated into northern hemisphere and southern hemisphere countries. Probably the simplest comparison is to determine whether the “typical” value for one group is above or below the “typical” value for the other; however much more detailed comparisons are possible and often needed. Virtually any of the distributional questions posed for one group in Chapters 2 can be asked of two or more groups in comparison to each other.