PCHART Statement: SHEWHART Procedure

Saving Control Limits

Note: See p Chart Examples in the SAS/QC Sample Library.

You can save the control limits for a p 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 17.60 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 17.65.

Figure 17.65: 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 $\sigma $ 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 17.66.

Figure 17.66: 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 p chart (not shown here) identical to the chart in Figure 17.60:

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: SHEWHART Procedure). For more information, see TABLE= Data Set.