Categorized Plots, 2D - Normal Probability Plots

This type of probability plot is constructed as follows. First, within each category, the values (observations) are rank ordered. From these ranks one can compute z values (i.e., standardized values of the normal distribution) based on the assumption that the data come from a normal distribution (see Computation Note ). These z values are plotted on the Y-axis in the plot. If the observed values (plotted on the X-axis) are normally distributed, then all values should fall onto a straight line. If the values are not normally distributed, then they will deviate from the line. Outliers may also become evident in this plot. If there is a general lack of fit, and the data seem to form a clear pattern (e.g., an S shape) around the line, then the variable may have to be transformed in some way (e.g., a log transformation to "pull-in" the tail of the distribution, etc.) before some statistical techniques that are affected by non-normality can be used.
For a detailed discussion of Categorized Graphs, see Categorized Graphs in Selected Topics in Graphical Analytic Techniques.