Comparative histograms are useful for comparing the distribution of a process variable across levels of classification variables. You can use the COMPHISTOGRAM statement to create one-way and two-way comparative histograms. When used with a single classification variable, the COMPHISTOGRAM statement displays an array of component histograms (stacked or side-by-side), one for each level of the classification variable. When used with two classification variables, the COMPHISTOGRAM statement displays a matrix of component histograms, one for each combination of levels of the classification variables.

In quality improvement applications, typical uses of comparative histograms include

• comparing the capability of a process before and after an improvement

• comparing process capabilities of two or more suppliers

• exploring stratification in process data due to different lots, machines, manufacturing methods, and so forth

• studying the evolution of process capability over successive time periods

You can use options in the COMPHISTOGRAM statement to

• specify the midpoints or endpoints for histogram intervals

• specify the number of rows and/or columns of component histograms

• display specification limits on the component histograms

• display density curves for fitted normal distributions

• display kernel density estimates

• request graphical enhancements

• inset summary statistics and process capability indices on the component histograms

You have two alternatives for producing comparative histograms with the COMPHISTOGRAM statement:

• Traditional graphics are produced by default.

• ODS Graphics output is produced when you specify the ODS GRAPHICS statement prior to the PROC CAPABILITY statement.

See Chapter 3, SAS/QC Graphics, for more information about producing these different kinds of graphs.

Note:You cannot use the COMPHISTOGRAM statement together with the CLASS statement.