Note: See Standard Deviation Chart (s Chart) Example 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 17.78 in a data set named Turblim
:
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 17.82.
Figure 17.82: The Data Set Turblim
Containing Control Limit Information
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 |
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 XSCHART 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:
proc shewhart data=Turbine; schart KWatts*Day / outtable=Turbtab nochart; run;
The data set Turbtab
is listed in Figure 17.83.
Figure 17.83: The OUTTABLE= Data Set Turbtab
Summary Statistics and Control Limit Information |
_VAR_ | Day | _SIGMAS_ | _LIMITN_ | _SUBN_ | _LCLS_ | _SUBS_ | _S_ | _UCLS_ | _STDDEV_ | _EXLIM_ |
---|---|---|---|---|---|---|---|---|---|---|
KWatts | 04JUL | 3 | 20 | 20 | 100.207 | 220.260 | 196.396 | 292.584 | 198.996 | |
KWatts | 05JUL | 3 | 20 | 20 | 100.207 | 210.427 | 196.396 | 292.584 | 198.996 | |
KWatts | 06JUL | 3 | 20 | 20 | 100.207 | 147.025 | 196.396 | 292.584 | 198.996 | |
KWatts | 07JUL | 3 | 20 | 20 | 100.207 | 157.637 | 196.396 | 292.584 | 198.996 | |
KWatts | 08JUL | 3 | 20 | 20 | 100.207 | 258.949 | 196.396 | 292.584 | 198.996 | |
KWatts | 09JUL | 3 | 20 | 20 | 100.207 | 211.566 | 196.396 | 292.584 | 198.996 | |
KWatts | 10JUL | 3 | 20 | 20 | 100.207 | 193.779 | 196.396 | 292.584 | 198.996 | |
KWatts | 11JUL | 3 | 20 | 20 | 100.207 | 212.024 | 196.396 | 292.584 | 198.996 | |
KWatts | 12JUL | 3 | 20 | 20 | 100.207 | 199.201 | 196.396 | 292.584 | 198.996 | |
KWatts | 13JUL | 3 | 20 | 20 | 100.207 | 173.455 | 196.396 | 292.584 | 198.996 | |
KWatts | 14JUL | 3 | 20 | 20 | 100.207 | 187.465 | 196.396 | 292.584 | 198.996 | |
KWatts | 15JUL | 3 | 20 | 20 | 100.207 | 205.472 | 196.396 | 292.584 | 198.996 | |
KWatts | 16JUL | 3 | 20 | 20 | 100.207 | 173.676 | 196.396 | 292.584 | 198.996 | |
KWatts | 17JUL | 3 | 20 | 20 | 100.207 | 200.576 | 196.396 | 292.584 | 198.996 | |
KWatts | 18JUL | 3 | 20 | 20 | 100.207 | 222.084 | 196.396 | 292.584 | 198.996 | |
KWatts | 19JUL | 3 | 20 | 20 | 100.207 | 185.724 | 196.396 | 292.584 | 198.996 | |
KWatts | 20JUL | 3 | 20 | 20 | 100.207 | 223.474 | 196.396 | 292.584 | 198.996 | |
KWatts | 21JUL | 3 | 20 | 20 | 100.207 | 145.267 | 196.396 | 292.584 | 198.996 | |
KWatts | 22JUL | 3 | 20 | 20 | 100.207 | 190.994 | 196.396 | 292.584 | 198.996 | |
KWatts | 23JUL | 3 | 20 | 20 | 100.207 | 208.858 | 196.396 | 292.584 | 198.996 |
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.
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 17.78:
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 Specialized Control Charts: SHEWHART Procedure). For more information, see TABLE= Data Set.