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CCHART Statement |
See SHWCCHR1 in the SAS/QC Sample Library |
You can save the control limits for a c chart in a SAS data set; this enables you to apply the control limits to future data (see "Reading Preestablished Control Limits" ) or subsequently modify the limits with a DATA step program.
The following statements read the data set TRUCKS
introduced in "Creating c Charts from Defect Count Data" and saves the control limit
information displayed in Figure 33.2 in a data set named DEFLIM:
proc shewhart data=trucks; cchart defects*truckid / outlimits=deflim nochart; run;
The OUTLIMITS= option names the data set containing the control
limits, and the NOCHART option suppresses the display of the chart.
Options such as OUTLIMITS= and NOCHART are
specified after the slash (/) in the CCHART statement. A complete
list of options is presented in the "Syntax" section. The data set DEFLIM is listed in
Figure 33.3.
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The data set DEFLIM contains one observation with the limits for
the process DEFECTS.
The variables _LCLC_, and _UCLC_ contain
the lower and upper control limits.
The variable
_C_ contains the central line, and the variable
_U_ contains the average number of nonconformities per inspection
unit. Since all the subgroups contain a single inspection unit, the
values of _C_ and _U_ are the same. 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 value of _U_ is an estimate or standard value.
For more information, see "OUTLIMITS= Data Set" .
Alternatively, you can use the OUTTABLE= option to
create an output data set
that saves both the control limits and the subgroup statistics,
as illustrated by the following statements:
title 'Number of Nonconformities and Control Limit Information'; proc shewhart data=trucks; cchart defects*truckid / outtable=trucktab nochart; run;
The OUTTABLE= data set TRUCKTAB is listed in Figure 33.4.
This data set contains one observation for each subgroup sample. The variables _SUBC_ and _SUBN_ contain the number of nonconformites per subgroup and the number of inspection units per subgroup. The variables _LCLC_ and _UCLC_ contain the lower and upper control limits, and the variable _C_ contains the central line. The variables _VAR_ and TRUCKID 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 in
the SHEWHART procedure. For example, the following statements
read TRUCKTAB and display a
c chart (not shown here) identical to the chart in Figure 33.2:
title 'c Chart for Paint Defects in New Trucks'; proc shewhart table=trucktab; cchart defects*truckid='*'; label _SUBC_ = 'Number of Paint Defects'; run;
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