Example 17.19 Specifying Standard Average Proportion

Note: See Specifying a Known Proportion for np Charts 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 an $np$ chart.

An $np$ chart is used to monitor the number of failing circuits in the data set Circuits, which is introduced in Creating np Charts from Count Data. The expected proportion of failing circuits is known to be $p_{0}=0.02$ . The following statements create an $np$ chart, shown in Output 17.19.1, using $p_0$ to compute the control limits:

title1 'np Chart for Failing Circuits';
title2 'Using Data in CIRCUITS and Standard Value P0=0.02';
proc shewhart data=Circuits;
   npchart Fail*Batch / subgroupn = 500
                        p0        = 0.02
                        npsymbol  = np0
                        nolegend
                        needles
                        odstitle  = title
                        odstitle2 = title2;
   label Batch ='Batch Number'
         Fail  ='Fraction Failing';
run;

Output 17.19.1: An $np$ Chart with Standard Value of $p_{0}$

The chart indicates that the process is in control. The P0= option specifies $p_0$ . The NPSYMBOL= option specifies a label for the central line indicating that the line represents a standard value. The NEEDLES option connects points to the central line with vertical needles. The NOLEGEND option suppresses the default legend for subgroup sample sizes. Labels for the vertical and horizontal axes are provided with the LABEL statement. For details concerning axis labeling, see Axis Labels.

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;

proc shewhart data=Circuits limits=Climits;
   npchart Fail*Batch / subgroupn = 500
                        npsymbol  = np0
                        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 17.19.1.

NPCHART Statement: SHEWHART Procedure

Example 17.19 Specifying Standard Average Proportion