Saving Control Limits

XSCHART Statement

Saving Control Limits

See SHWXS1 in the SAS/QC Sample Library

You can save the control limits for $\bar{X}$ and s charts 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 Charts for Means and Standard Deviations from Raw Data" ) and save the control limits displayed in Figure 44.2 in a data set named TURBLIM:

   title1 'Control Limits for Power Output Data';
   proc shewhart data=turbine;
      xschart 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 charts. The data set TURBLIM is listed in Figure 44.6.

Control Limits for Power Output Data

_VAR_	_SUBGRP_	_TYPE_	_LIMITN_	_ALPHA_	_SIGMAS_	_LCLX_	_MEAN_	_UCLX_	_LCLS_	_S_	_UCLS_	_STDDEV_
kwatts	day	ESTIMATE	20	.002699796	3	3351.92	3485.41	3618.90	100.207	196.396	292.584	198.996

Figure 44.6: The Data Set TURBLIM Containing Control Limit Information

The data set TURBLIM contains one observation with the limits for process KWATTS. The variables _LCLX_ and _UCLX_ contain the lower and upper control limits for the $\bar{X}$ chart, and the variables _LCLS_ and _UCLS_ contain the lower and upper control limits for the s chart. The variable _MEAN_ contains the central line for the $\bar{X}$ chart, and the variable _S_ contains the central line for the s chart. The value of _MEAN_ is an estimate of the process mean, and the value of _STDDEV_ is an estimate of the process standard deviation $\sigma$ .The value of _LIMITN_ is the nominal sample size associated with the control limits, and the value of _SIGMAS_ is the multiple of $\sigma$ 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. 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;
      xschart kwatts*day / outtable=turbtab
                           nochart;
   run;

The data set TURBTAB contains one observation for each subgroup sample. The variables _SUBX_, _SUBS_, and _SUBN_ contain the subgroup means, subgroup standard deviations, and subgroup sample sizes. The variables _LCLX_ and _UCLX_ contain the lower and upper control limits for the $\bar{X}$ chart. The variables _LCLS_ and _UCLS_ contain the lower and upper control limits for the s chart. The variable _MEAN_ contains the central line for the $\bar{X}$ chart. The variable _S_ contains the central line for the s chart. The variables _VAR_ and BATCH contain the process name and values of the subgroup-variable, respectively. For more information, see "OUTTABLE= Data Set" .

The data set TURBTAB is listed in Figure 44.7.

Summary Statistics and Control Limit Information

_VAR_	day	_SIGMAS_	_LIMITN_	_SUBN_	_LCLX_	_SUBX_	_MEAN_	_UCLX_	_EXLIM_	_LCLS_	_SUBS_	_S_	_UCLS_	_EXLIMS_
kwatts	04JUL	3	20	20	3351.92	3487.40	3485.41	3618.90		100.207	220.260	196.396	292.584
kwatts	05JUL	3	20	20	3351.92	3471.65	3485.41	3618.90		100.207	210.427	196.396	292.584
kwatts	06JUL	3	20	20	3351.92	3488.30	3485.41	3618.90		100.207	147.025	196.396	292.584
kwatts	07JUL	3	20	20	3351.92	3434.20	3485.41	3618.90		100.207	157.637	196.396	292.584
kwatts	08JUL	3	20	20	3351.92	3475.80	3485.41	3618.90		100.207	258.949	196.396	292.584
kwatts	09JUL	3	20	20	3351.92	3518.10	3485.41	3618.90		100.207	211.566	196.396	292.584
kwatts	10JUL	3	20	20	3351.92	3492.65	3485.41	3618.90		100.207	193.779	196.396	292.584
kwatts	11JUL	3	20	20	3351.92	3496.40	3485.41	3618.90		100.207	212.024	196.396	292.584
kwatts	12JUL	3	20	20	3351.92	3398.50	3485.41	3618.90		100.207	199.201	196.396	292.584
kwatts	13JUL	3	20	20	3351.92	3456.05	3485.41	3618.90		100.207	173.455	196.396	292.584
kwatts	14JUL	3	20	20	3351.92	3493.60	3485.41	3618.90		100.207	187.465	196.396	292.584
kwatts	15JUL	3	20	20	3351.92	3563.30	3485.41	3618.90		100.207	205.472	196.396	292.584
kwatts	16JUL	3	20	20	3351.92	3519.05	3485.41	3618.90		100.207	173.676	196.396	292.584
kwatts	17JUL	3	20	20	3351.92	3474.20	3485.41	3618.90		100.207	200.576	196.396	292.584
kwatts	18JUL	3	20	20	3351.92	3443.60	3485.41	3618.90		100.207	222.084	196.396	292.584
kwatts	19JUL	3	20	20	3351.92	3586.35	3485.41	3618.90		100.207	185.724	196.396	292.584
kwatts	20JUL	3	20	20	3351.92	3486.45	3485.41	3618.90		100.207	223.474	196.396	292.584
kwatts	21JUL	3	20	20	3351.92	3492.90	3485.41	3618.90		100.207	145.267	196.396	292.584
kwatts	22JUL	3	20	20	3351.92	3432.80	3485.41	3618.90		100.207	190.994	196.396	292.584
kwatts	23JUL	3	20	20	3351.92	3496.90	3485.41	3618.90		100.207	208.858	196.396	292.584

Figure 44.7: The OUTTABLE= Data Set TURBTAB

A data set created with the OUTTABLE= option can be read later as a TABLE= data set. For example, the following statements read TURBTAB and display charts (not shown here) identical to those in Figure 44.2:

   title 'Mean and Standard Deviation Charts for Power Output';
   proc shewhart table=turbtab;
      xschart 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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