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| NPCHART Statement |
| See SHWNP1 in the SAS/QC Sample Library |
You can save the control limits for an np 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 np Charts from Count Data" ) and save the control limits displayed in Figure 37.2 in a data set named CIRLIM:
title 'Control Limits for the Number of Failing Circuits';
proc shewhart data=circuits;
npchart 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 37.6.
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The data set CIRLIM contains one observation with the limits for
process FAIL. The variables _LCLNP_ and _UCLNP_ contain the
lower and upper control limits, and the variable
_NP_ contains the central line.
The variable _P_ contains the average proportion of nonconforming
items. 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
a 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:
title 'Number Nonconforming and Control Limit Information';
proc shewhart data=circuits;
npchart fail*batch / subgroupn=500
outtable=cirtable
nochart;
run;
The data set CIRTABLE is listed in Figure 37.7.
This data set contains one observation for each subgroup sample. The variables _SUBNP_ and _SUBN_ contain the subgroup numbers of nonconforming items and subgroup sample sizes, respectively. The variables _LCLNP_ and _UCLNP_ contain the lower and upper control limits, and the variable _NP_ 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 CIRTABLE and display an np chart (not shown here) identical to the chart in Figure 37.2:
title 'np Chart for the Number of Failing Circuits';
proc shewhart table=cirtable;
npchart 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 Chapter 49, "Specialized Control Charts"). For more information, see "TABLE= Data Set" .
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