Multiple groups of individuals

Sometimes we know that the individuals belong to two or more groups before the data are collected or, equivalently, that they have different values of an extra categorical variable.

Information about groups is best displayed by plotting the separate groups against a common axis.

Back-to-back stem and leaf plots

Stem and leaf plots can be used to compare two groups of individuals, if drawn on different sides of a common column of stems. (They are less useful if there are three or more groups.)