# Normal Probability Plots

A Normal probability plot provides a simple way to tell whether the numbers in a variable are approximately normally distributed.

Data Desk sorts the values in the variable and then, starting with the smallest value, poses the question: “If this were a sample from the standard normal distribution (that is, the normal distribution with zero mean and unit variance), what would I expect the smallest value to be?” The answer is the first normal score. It depends only on the number of cases in the variable and can be estimated with the *NScores* function available in derived variables. The probability plot graphs the observed smallest value against the value expected under the assumption of a normal distribution.

The question is repeated for the second smallest value: “If this were a sample from the standard normal distribution what would I expect the *second* smallest value to be?” The observed second smallest value is plotted against the expected second smallest value.

To make a normal probability plot, select the variable to plot and choose **Plot > Normal Prob Plots**. Normal probability plots are also available from HyperView menus in several procedures in which the assumption of normality is important.