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SCHART Statement |
See SHWSCHR in the SAS/QC Sample Library |
You can save the control limits for an s 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 measurements from the data set TURBINE (see "Creating Standard Deviation Charts from Raw Data" ) and save the control limits displayed in Figure 40.2 in a data set named TURBLIM:
title 'Control Limits for Power Output Data'; proc shewhart data=turbine; schart kwatts*day / outlimits=turblim 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 TURBLIM is listed in Figure 40.6.
The data set TURBLIM contains one observation with the limits for process KWATTS. The variables _LCLS_ and _UCLS_ contain the lower and upper control limits, and the variable _S_ contains the central line. The value of _MEAN_ is an estimate of the process mean, and the value of _STDDEV_ is an estimate of the process standard deviation .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 values of _MEAN_ and _STDDEV_ are estimates or standard values. The variables _LCLX_ and _UCLX_, which contain the lower and upper control limits for subgroup means, are included so that the data set TURBLIM can be used to create an chart (see Chapter 44, "XSCHART Statement"). 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 'Summary Statistics and Control Limit Information'; proc shewhart data=turbine; schart kwatts*day / outtable=turbtab nochart; run;The data set TURBTAB is listed in Figure 40.7.
This data set contains one observation for each subgroup sample. The variables _SUBS_ and _SUBN_ contain the subgroup standard deviations and subgroup sample sizes. The variables _LCLS_ and _UCLS_ contain the lower and upper control limits, and the variable _S_ 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" .
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An OUTTABLE= data set can be read later as a TABLE= data set.
For example, the following statements read
TURBTAB and display an s chart
(not shown here) identical to the chart in Figure 40.2:
title 'Chart for Standard Deviations of Power Output'; symbol v=dot; proc shewhart table=turbtab; schart kwatts*day; run;
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