Example 13.28 Specifying Control Limit Information

[See SHWREX2 in the SAS/QC Sample Library]This example illustrates how you can use a DATA step program to create a LIMITS= data set. You can provide previously established values for the limits and central line with the variables _LCLR_, _R_, and _UCLR_, as in the following statements:

data Dlimits2;
   length _var_ _subgrp_ _type_ $8;
   _var_    = 'Time';
   _subgrp_ = 'Lot';
   _type_   = 'STANDARD';
   _limitn_ = 6;
   _lclr_   = .03;
   _r_      = .12;
   _uclr_   = .25;
run;

The following statements¹ apply the control limits in Dlimits2 to the measurements in DISKS2 (see Reading Preestablished Control Limits) and create the chart shown in Output 13.28.1:

ods graphics on;
title 'Specifying Control Limit Information';
proc shewhart data=Disks2 limits=Dlimits2;
   rchart Time*Lot / odstitle=title;
run;

Output 13.28.1 Reading Control Limits from Dlimits2

In some cases, a standard value () may be available for the process standard deviation. The following DATA step creates a data set named Dlimits3 that provides this value:

data Dlimits3;
   length _var_ _subgrp_ _type_ $8;
   _var_    = 'Time';
   _subgrp_ = 'Lot';
   _stddev_ = .045;
   _limitn_ = 6;
   _type_   = 'STDSIGMA';
run;

The variable _TYPE_ is a bookkeeping variable whose value indicates that the value of _STDDEV_ is a standard value rather than an estimate.

The following statements read the value of from Dlimits3 and create the chart shown in Output 13.28.2:

title 'Specifying Control Limit Information';
proc shewhart data=Disks2 limits=Dlimits3;
   rchart Time*Lot / nolimit0 odstitle=title;
run;
ods graphics off;

The NOLIMIT0 option suppresses the display of a fixed lower control limit if the value of the limit is zero (which is the case in this example).

Output 13.28.2 Reading in Standard Value for Process Standard Deviation

Instead of specifying with the variable _STDDEV_ in a LIMITS= data set, you can use the SIGMA0= option in the RCHART statement. The following statements create an chart identical to the chart shown in Output 13.28.2:

proc shewhart data=Disks;
   rchart Time*Lot / sigma0=.045 nolimit0;
run;

For more information, see LIMITS= Data Set.