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 SCHART Statement

## Saving Control Limits

 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.

 Control Limits for Power Output Data

 _VAR_ _SUBGRP_ _TYPE_ _LIMITN_ _ALPHA_ _SIGMAS_ _LCLX_ _MEAN_ _UCLX_ _LCLS_ _S_ _UCLS_ _STDDEV_ kwatts day ESTIMATE 20 .002792725 3 3351.92 3485.41 3618.90 100.207 196.396 292.584 198.996
Figure 40.6: The Data Set TURBLIM Containing Control Limit Information

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" .

 Summary Statistics and Control Limit Information

 _VAR_ day _SIGMAS_ _LIMITN_ _SUBN_ _LCLS_ _SUBS_ _S_ _UCLS_ _EXLIM_ kwatts 04JUL 3 20 20 100.207 220.260 196.396 292.584 kwatts 05JUL 3 20 20 100.207 210.427 196.396 292.584 kwatts 06JUL 3 20 20 100.207 147.025 196.396 292.584 kwatts 07JUL 3 20 20 100.207 157.637 196.396 292.584 kwatts 08JUL 3 20 20 100.207 258.949 196.396 292.584 kwatts 09JUL 3 20 20 100.207 211.566 196.396 292.584 kwatts 10JUL 3 20 20 100.207 193.779 196.396 292.584 kwatts 11JUL 3 20 20 100.207 212.024 196.396 292.584 kwatts 12JUL 3 20 20 100.207 199.201 196.396 292.584 kwatts 13JUL 3 20 20 100.207 173.455 196.396 292.584 kwatts 14JUL 3 20 20 100.207 187.465 196.396 292.584 kwatts 15JUL 3 20 20 100.207 205.472 196.396 292.584 kwatts 16JUL 3 20 20 100.207 173.676 196.396 292.584 kwatts 17JUL 3 20 20 100.207 200.576 196.396 292.584 kwatts 18JUL 3 20 20 100.207 222.084 196.396 292.584 kwatts 19JUL 3 20 20 100.207 185.724 196.396 292.584 kwatts 20JUL 3 20 20 100.207 223.474 196.396 292.584 kwatts 21JUL 3 20 20 100.207 145.267 196.396 292.584 kwatts 22JUL 3 20 20 100.207 190.994 196.396 292.584 kwatts 23JUL 3 20 20 100.207 208.858 196.396 292.584
Figure 40.7: The OUTTABLE= Data Set TURBTAB

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;


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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