Saving Control Limits

RCHART Statement

Saving Control Limits

See SHWRCHR in the SAS/QC Sample Library

You can save the control limits for an R 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 DISKS (see "Creating Range Charts from Raw Data" ) and save the control limits displayed in Figure 39.2 in a data set named DISKLIM:

   title 'Control Limits for Disk Times';
   proc shewhart data=disks;
      rchart time*lot / outlimits = disklim
                        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 DISKLIM is listed in Figure 39.6.

Control Limits for Disk Times

_VAR_	_SUBGRP_	_TYPE_	_LIMITN_	_ALPHA_	_SIGMAS_	_LCLX_	_MEAN_	_UCLX_	_LCLR_	_R_	_UCLR_	_STDDEV_
time	lot	ESTIMATE	6	.004447667	3	7.94314	8.00307	8.06299	0	0.124	0.24847	0.048927

Figure 39.6: The Data Set DISKLIM Containing Control Limit Information

The data set DISKLIM contains one observation with the limits for process TIME. The variables _LCLR_ and _UCLR_ contain the lower and upper control limits, and the variable _R_ 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 $\sigma$ .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. The variables _LCLX_ and _UCLX_, which contain the lower and upper control limits for subgroup means, are included so that the data set DISKLIM can be used to create an $\bar{X}$ chart (see Chapter 43, "XRCHART 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=disks;
     rchart time*lot / outtable=disktab
                       nochart;
   run;

The data set DISKTAB is listed in Figure 39.7.

Summary Statistics and Control Limit Information

_VAR_	lot	_SIGMAS_	_LIMITN_	_SUBN_	_LCLR_	_SUBR_	_R_	_UCLR_	_EXLIM_
time	1	3	6	6	0	0.16	0.124	0.24847
time	2	3	6	6	0	0.09	0.124	0.24847
time	3	3	6	6	0	0.07	0.124	0.24847
time	4	3	6	6	0	0.10	0.124	0.24847
time	5	3	6	6	0	0.16	0.124	0.24847
time	6	3	6	6	0	0.12	0.124	0.24847
time	7	3	6	6	0	0.15	0.124	0.24847
time	8	3	6	6	0	0.11	0.124	0.24847
time	9	3	6	6	0	0.15	0.124	0.24847
time	10	3	6	6	0	0.10	0.124	0.24847
time	11	3	6	6	0	0.10	0.124	0.24847
time	12	3	6	6	0	0.09	0.124	0.24847
time	13	3	6	6	0	0.13	0.124	0.24847
time	14	3	6	6	0	0.10	0.124	0.24847
time	15	3	6	6	0	0.13	0.124	0.24847
time	16	3	6	6	0	0.09	0.124	0.24847
time	17	3	6	6	0	0.11	0.124	0.24847
time	18	3	6	6	0	0.20	0.124	0.24847
time	19	3	6	6	0	0.18	0.124	0.24847
time	20	3	6	6	0	0.10	0.124	0.24847
time	21	3	6	6	0	0.14	0.124	0.24847
time	22	3	6	6	0	0.20	0.124	0.24847
time	23	3	6	6	0	0.11	0.124	0.24847
time	24	3	6	6	0	0.06	0.124	0.24847
time	25	3	6	6	0	0.15	0.124	0.24847

Figure 39.7: The Data Set DISKTAB

This data set contains one observation for each subgroup sample. The variables _SUBR_ and _SUBN_ contain the subgroup ranges and subgroup sample sizes. The variables _LCLR_ and _UCLR_ contain the lower and upper control limits, and the variable _R_ 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" . An OUTTABLE= data set can be read later as a TABLE= data set. For example, the following statements read DISKTAB and display an R chart (not shown here) identical to the chart in Figure 39.2:

   title 'Range Chart for Disk Drive Test Times';
   proc shewhart table=disktab;
      rchart time*lot;
   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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