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PCHART Statement |
See SHWPEX2 in the SAS/QC Sample Library |
In some situations, a standard (known) value (p_{0}) is available for the expected proportion of nonconforming items, based on extensive testing or previous sampling. This example illustrates how you can specify p_{0} to create a p chart.
A p chart is used to monitor the proportion of failing circuits in the data set CIRCUITS, which is introduced at "Creating p Charts from Count Data" . The expected proportion is known to be p_{0}=0.02. The following statements create a p chart, shown in Output 38.2.1, using p_{0} to compute the control limits:
title1 'p Chart for Failing Circuits'; title2 'Using Data in CIRCUITS and Standard Value P0=0.02'; symbol v=star c=salmon; proc shewhart data=circuits; pchart fail*batch / subgroupn = 500 p0 = 0.02 psymbol = p0 nolegend needles cframe = lib cinfill = bwh cneedles = salmon; label batch = 'Batch Number' fail = 'Fraction Failing'; run;
Alternatively, you can specify p_{0} using the variable _P_
in a LIMITS= data set,^{*}
as follows:
data climits; length _var_ _subgrp_ _type_ $8; _p_ = 0.02; _subgrp_ = 'batch'; _var_ = 'fail'; _type_ = 'STANDARD'; _limitn_ = 500; run; proc shewhart data=circuits limits=climits; pchart fail*batch / subgroupn = 500 psymbol = p0 nolegend needles; label batch ='Batch Number' fail ='Fraction Failing'; run;
The bookkeeping variable _TYPE_ indicates that _P_ has a standard value. The chart produced by these statements is identical to the chart in Output 38.2.1.
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