QQPLOT Statement

Overview

The QQPLOT statement creates a quantile-quantile plot (Q-Q plot), which compares ordered values of a variable with quantiles of a specified theoretical distribution such as the normal. If the data distribution matches the theoretical distribution, the points on the plot form a linear pattern. Thus, you can use a Q-Q plot to determine how well a theoretical distribution models a set of measurements.

You can specify one of the following theoretical distributions with the QQPLOT statement:

beta
exponential
gamma
three-parameter lognormal
normal
two-parameter Weibull
three-parameter Weibull

You can use options in the QQPLOT statement to

specify or estimate parameters for the theoretical distribution
display a reference line corresponding to specific location and scale parameters for the theoretical distribution
request graphical enhancements

Note: Q-Q plots are similar to probability plots, which you can create with the PROBPLOT statement (see Chapter 9, "PROBPLOT Statement"). Q-Q plots are preferable for graphical estimation of distribution parameters and capability indices, whereas probability plots are preferable for graphical estimation of percentiles.

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