Overview: COMPHISTOGRAM Statement

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:

  • ODS Graphics output is produced if ODS Graphics is enabled, for example by specifying the ODS GRAPHICS ON statement prior to the PROC statement.

  • Otherwise, traditional graphics are produced if SAS/GRAPH® is licensed.

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.