Saving Control Limits

[See SHWSCHR in the SAS/QC Sample Library]You can save the control limits for an chart in a SAS data set; this enables you to apply the control limits to future data (see Reading Preestablished Control Limits) or modify the limits with a DATA step program.

The following statements read measurements from the data set Turbine (see Creating Standard Deviation Charts from Raw Data) and save the control limits displayed in Figure 13.28.4 in a data set named Turblim:

proc shewhart data=Turbine;
   schart KWatts*Day / outlimits=Turblim
                       nochart;
run;

The OUTLIMITS= option names the data set containing the control limits, and the NOCHART option suppresses the display of the chart. The data set Turblim is listed in Figure 13.28.8.

The data set Turblim contains one observation with the limits for process KWatts. The variables _LCLS_ and _UCLS_ contain the lower and upper control limits, and the variable _S_ contains the central line. The value of _MEAN_ is an estimate of the process mean, and the value of _STDDEV_ is an estimate of the process standard deviation . The value of _LIMITN_ is the nominal sample size associated with the control limits, and the value of _SIGMAS_ is the multiple of associated with the control limits. The variables _VAR_ and _SUBGRP_ are bookkeeping variables that save the process and subgroup-variable. The variable _TYPE_ is a bookkeeping variable that indicates whether the values of _MEAN_ and _STDDEV_ are estimates or standard values. The variables _LCLX_ and _UCLX_, which contain the lower and upper control limits for subgroup means, are included so that the data set Turblim can be used to create an chart (see XSCHART Statement). For more information, see OUTLIMITS= Data Set.

You can create an output data set containing both control limits and summary statistics with the OUTTABLE= option, as illustrated by the following statements:

proc shewhart data=Turbine;
   schart KWatts*Day / outtable=Turbtab
                       nochart;
run;

The data set Turbtab is listed in Figure 13.28.9.

This data set contains one observation for each subgroup sample. The variables _SUBS_ and _SUBN_ contain the subgroup standard deviations and subgroup sample sizes. The variables _LCLS_ and _UCLS_ contain the lower and upper control limits, and the variable _S_ contains the central line. The variables _VAR_ and Batch contain the process name and values of the subgroup-variable, respectively. For more information, see OUTTABLE= Data Set.

An OUTTABLE= data set can be read later as a TABLE= data set. For example, the following statements read Turbtab and display an chart (not shown here) identical to the chart in Figure 13.28.4:

title 'Chart for Standard Deviations of Power Output';
symbol v=dot;
proc shewhart table=Turbtab;
   schart KWatts*Day;
run;

Because the SHEWHART procedure simply displays the information in a TABLE= data set, you can use TABLE= data sets to create specialized control charts (see Specialized Control Charts). For more information, see TABLE= Data Set.