SCHART Statement: SHEWHART Procedure

Overview: SCHART Statement

The SCHART statement creates an s chart for subgroup standard deviations, which is used to analyze the variability of a process.[35]

You can use options in the SCHART statement to

  • compute control limits from the data based on a multiple of the standard error of the plotted standard deviations or as probability limits

  • tabulate subgroup sample sizes, subgroup standard deviations, control limits, and other information

  • save control limits in an output data set

  • save subgroup sample sizes, subgroup means, and subgroup standard deviations in an output data set

  • read preestablished control limits from a data set

  • specify a method for estimating the process standard deviation

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

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

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

  • superimpose stars at points to represent related multivariate factors

  • 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 s charts with the SCHART 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.


[35] You can also use R charts for this purpose; see RCHART Statement: SHEWHART Procedure. In general, s charts are recommended with large subgroup sample sizes ($n_ i \geq 10$).