BOXCHART Statement: SHEWHART Procedure

Overview: BOXCHART Statement

The BOXCHART statement creates an $\bar{X}$ 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 18.2 and Example 18.3).

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:

  • 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 by default if SAS/GRAPH® is licensed.

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

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