The BOXCHART statement creates an chart for subgroup means superimposed with box-and-whisker plots of the measurements in each subgroup. Throughout this chapter, a chart of this type is referred to as a box chart. This chart is recommended for large subgroup sample sizes (typically greater than ten). You can also use the BOXCHART statement to create standard side-by-side box-and-whisker plots (see Example 13.2 and Example 13.3).

You can use options in the BOXCHART statement to

• specify control limits for subgroup means or medians

• compute control limits from the data based on a multiple of the standard error of the means (or medians) or as probability limits

• tabulate subgroup summary statistics and control limits

• save control limits in an output data set

• save subgroup summary statistics in an output data set

• read preestablished control limits from a data set

• apply tests for special causes (also known as runs tests and Western Electric rules)

• specify one of several methods for estimating the process standard deviation

• specify whether subgroup standard deviations or subgroup ranges are used to estimate the process standard deviation

• specify a known (standard) process mean and standard deviation for computing control limits

• create a secondary chart that displays a time trend removed from the data (see Displaying Trends in Process Data)

• specify one of several methods for calculating quantile statistics (percentiles)

• control the style of the box-and-whisker plots

• display distinct sets of control limits for data from successive time phases

• add block legends and symbol markers to reveal stratification in process data

• clip extreme points to make the chart more readable

• display vertical and horizontal reference lines

• control axis values and labels

• control layout and appearance of the chart

You have three alternatives for producing box charts with the BOXCHART statement:

• Traditional graphics are produced by default.

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

• Legacy line printer charts are produced when you specify the LINEPRINTER option in the PROC SHEWHART statement.

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