RCHART Statement: SHEWHART Procedure

Saving Control Limits

Note: See Range Chart (R Chart) Examples 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 17.71 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 17.75.

Figure 17.75: The Data Set Disklim Containing Control Limit Information

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



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 XRCHART Statement: SHEWHART Procedure). 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 17.76.

Figure 17.76: The Data Set Disktab

Summary Statistics and Control Limit Information

_VAR_ Lot _SIGMAS_ _LIMITN_ _SUBN_ _LCLR_ _SUBR_ _R_ _UCLR_ _STDDEV_ _EXLIM_
Time 1 3 6 6 0 0.16 0.124 0.24847 0.048927  
Time 2 3 6 6 0 0.09 0.124 0.24847 0.048927  
Time 3 3 6 6 0 0.07 0.124 0.24847 0.048927  
Time 4 3 6 6 0 0.10 0.124 0.24847 0.048927  
Time 5 3 6 6 0 0.16 0.124 0.24847 0.048927  
Time 6 3 6 6 0 0.12 0.124 0.24847 0.048927  
Time 7 3 6 6 0 0.15 0.124 0.24847 0.048927  
Time 8 3 6 6 0 0.11 0.124 0.24847 0.048927  
Time 9 3 6 6 0 0.15 0.124 0.24847 0.048927  
Time 10 3 6 6 0 0.10 0.124 0.24847 0.048927  
Time 11 3 6 6 0 0.10 0.124 0.24847 0.048927  
Time 12 3 6 6 0 0.09 0.124 0.24847 0.048927  
Time 13 3 6 6 0 0.13 0.124 0.24847 0.048927  
Time 14 3 6 6 0 0.10 0.124 0.24847 0.048927  
Time 15 3 6 6 0 0.13 0.124 0.24847 0.048927  
Time 16 3 6 6 0 0.09 0.124 0.24847 0.048927  
Time 17 3 6 6 0 0.11 0.124 0.24847 0.048927  
Time 18 3 6 6 0 0.20 0.124 0.24847 0.048927  
Time 19 3 6 6 0 0.18 0.124 0.24847 0.048927  
Time 20 3 6 6 0 0.10 0.124 0.24847 0.048927  
Time 21 3 6 6 0 0.14 0.124 0.24847 0.048927  
Time 22 3 6 6 0 0.20 0.124 0.24847 0.048927  
Time 23 3 6 6 0 0.11 0.124 0.24847 0.048927  
Time 24 3 6 6 0 0.06 0.124 0.24847 0.048927  
Time 25 3 6 6 0 0.15 0.124 0.24847 0.048927  



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 17.71:

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 Specialized Control Charts: SHEWHART Procedure). For more information, see TABLE= Data Set.