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UCHART Statement |
See SHWUCHR in the SAS/QC Sample Library |
You can save the control limits for a u 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 defect counts from the
data set FABRIC (see
"Creating u Charts from Defect Count Data" )
and save the control limits
displayed in Figure 41.2 in a data set named FABLIM:
title 'Control Limits Data Set FABLIM'; proc shewhart data=fabric; uchart defects*roll / subgroupn=30 outlimits=fablim nochart; run;
The SUBGROUPN= option specifies the number of inspection units in each subgroup sample. The OUTLIMITS= option names the data set containing the control limits, and the NOCHART option suppresses the display of the chart. The data set FABLIM is listed in Figure 41.3.
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The data set FABLIM contains one observation with the limits for process DEFECTS. The variables _LCLU_ and _UCLU_ contain the lower and upper control limits, and the variable _U_ 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 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 _U_ is an estimate or standard value. For more information, see "OUTLIMITS= Data Set" .
Alternatively, you can use the OUTTABLE= option to create an output data set that saves both the control limits and the subgroup statistics, as illustrated by the following statements:
title 'Number of Defects Per Square Meter and Control Limits'; proc shewhart data=fabric; uchart defects*roll / subgroupn=30 outtable=fabtab nochart; run;
This data set contains one observation for each subgroup sample. The variables _SUBU_ and _SUBN_ contain the number of nonconformities per unit in each subgroup and the number of inspection units per subgroup. The variables _LCLU_ and _UCLU_ contain the lower and upper control limits, and the variable _U_ contains the central line. The variables _VAR_ and ROLL 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 by the SHEWHART procedure. For example, the following statements read FABTAB and display a u chart (not shown here) identical to the chart in Figure 41.2:
title 'u Chart for Fabric Defects'; proc shewhart table=fabtab; uchart defects*roll / subgroupn=30; 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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