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The SHEWHART Procedure

Saving Control Limits

[See SHWPCHR in the SAS/QC Sample Library]You can save the control limits for a 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 the number of nonconforming items per subgroup from the data set Circuits (see Creating p Charts from Count Data) and save the control limits displayed in Figure 13.21.4 in a data set named Cirlim:

proc shewhart data=Circuits;
   pchart Fail*Batch / subgroupn = 500
                       outlimits = Cirlim
                       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 Cirlim is listed in Figure 13.21.9.

Output 13.21.9 The Data Set Cirlim Containing Control Limit Information
Control Limits for the Proportion of Failing Circuits

_VAR_ _SUBGRP_ _TYPE_ _LIMITN_ _ALPHA_ _SIGMAS_ _LCLP_ _P_ _UCLP_
Fail Batch ESTIMATE 500 .005040334 3 .000930786 0.019467 0.038003

The data set Cirlim contains one observation with the limits for process Fail. The variables _LCLP_ and _UCLP_ contain the lower and upper control limits, and the variable _P_ contains the central line. 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 value of _P_ is an estimate or standard value.

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=Circuits;
   pchart Fail*Batch / subgroupn = 500
                       outtable  = Cirtable
                       nochart ;
run;

The data set Cirtable is listed in Figure 13.21.10.

Output 13.21.10 The Data Set Cirtable
Subgroup Proportions and Control Limit Information

_VAR_ Batch _SIGMAS_ _LIMITN_ _SUBN_ _LCLP_ _SUBP_ _P_ _UCLP_ _EXLIM_
Fail 1 3 500 500 .000930786 0.010 0.019467 0.038003  
Fail 2 3 500 500 .000930786 0.012 0.019467 0.038003  
Fail 3 3 500 500 .000930786 0.022 0.019467 0.038003  
Fail 4 3 500 500 .000930786 0.012 0.019467 0.038003  
Fail 5 3 500 500 .000930786 0.008 0.019467 0.038003  
Fail 6 3 500 500 .000930786 0.018 0.019467 0.038003  
Fail 7 3 500 500 .000930786 0.034 0.019467 0.038003  
Fail 8 3 500 500 .000930786 0.020 0.019467 0.038003  
Fail 9 3 500 500 .000930786 0.024 0.019467 0.038003  
Fail 10 3 500 500 .000930786 0.018 0.019467 0.038003  
Fail 11 3 500 500 .000930786 0.016 0.019467 0.038003  
Fail 12 3 500 500 .000930786 0.014 0.019467 0.038003  
Fail 13 3 500 500 .000930786 0.014 0.019467 0.038003  
Fail 14 3 500 500 .000930786 0.030 0.019467 0.038003  
Fail 15 3 500 500 .000930786 0.016 0.019467 0.038003  
Fail 16 3 500 500 .000930786 0.036 0.019467 0.038003  
Fail 17 3 500 500 .000930786 0.024 0.019467 0.038003  
Fail 18 3 500 500 .000930786 0.032 0.019467 0.038003  
Fail 19 3 500 500 .000930786 0.008 0.019467 0.038003  
Fail 20 3 500 500 .000930786 0.014 0.019467 0.038003  
Fail 21 3 500 500 .000930786 0.034 0.019467 0.038003  
Fail 22 3 500 500 .000930786 0.024 0.019467 0.038003  
Fail 23 3 500 500 .000930786 0.016 0.019467 0.038003  
Fail 24 3 500 500 .000930786 0.014 0.019467 0.038003  
Fail 25 3 500 500 .000930786 0.030 0.019467 0.038003  
Fail 26 3 500 500 .000930786 0.012 0.019467 0.038003  
Fail 27 3 500 500 .000930786 0.016 0.019467 0.038003  
Fail 28 3 500 500 .000930786 0.024 0.019467 0.038003  
Fail 29 3 500 500 .000930786 0.014 0.019467 0.038003  
Fail 30 3 500 500 .000930786 0.018 0.019467 0.038003  

This data set contains one observation for each subgroup sample. The variables _SUBP_ and _SUBN_ contain the subgroup proportions of nonconforming items and subgroup sample sizes. The variables _LCLP_ and _UCLP_ contain the lower and upper control limits, and the variable _P_ 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 the information in Cirtable and display a chart (not shown here) identical to the chart in Figure 13.21.4:

title 'p Chart for the Proportion of Failing Circuits';
proc shewhart table=Cirtable;
   pchart Fail*Batch;
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.

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