CCHART Statement: SHEWHART Procedure
Note: See c Chart Examples 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 the section 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 17.16 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 section Syntax: CCHART Statement. The data set Deflim is listed in Figure 17.17.
Figure 17.17: The Data Set Deflim Containing Control Limit Information
| Control Limits Data Set Deflim |
| _VAR_ | _SUBGRP_ | _TYPE_ | _LIMITN_ | _ALPHA_ | _SIGMAS_ | _U_ | _LCLC_ | _C_ | _UCLC_ |
|---|---|---|---|---|---|---|---|---|---|
| Defects | TruckID | ESTIMATE | 1 | .002902622 | 3 | 7.8 | 0 | 7.8 | 16.1785 |
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 the section 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 17.18.
Figure 17.18: The Data Set Trucktab
| Number of Nonconformities and Control Limit Information |
| _VAR_ | TruckID | _SIGMAS_ | _LIMITN_ | _SUBN_ | _LCLC_ | _SUBC_ | _C_ | _UCLC_ | _EXLIM_ |
|---|---|---|---|---|---|---|---|---|---|
| Defects | C1 | 3 | 1 | 1 | 0 | 5 | 7.8 | 16.1785 | |
| Defects | C2 | 3 | 1 | 1 | 0 | 4 | 7.8 | 16.1785 | |
| Defects | C3 | 3 | 1 | 1 | 0 | 4 | 7.8 | 16.1785 | |
| Defects | C4 | 3 | 1 | 1 | 0 | 8 | 7.8 | 16.1785 | |
| Defects | C5 | 3 | 1 | 1 | 0 | 7 | 7.8 | 16.1785 | |
| Defects | C6 | 3 | 1 | 1 | 0 | 12 | 7.8 | 16.1785 | |
| Defects | C7 | 3 | 1 | 1 | 0 | 3 | 7.8 | 16.1785 | |
| Defects | C8 | 3 | 1 | 1 | 0 | 11 | 7.8 | 16.1785 | |
| Defects | E4 | 3 | 1 | 1 | 0 | 8 | 7.8 | 16.1785 | |
| Defects | E9 | 3 | 1 | 1 | 0 | 4 | 7.8 | 16.1785 | |
| Defects | E7 | 3 | 1 | 1 | 0 | 9 | 7.8 | 16.1785 | |
| Defects | E6 | 3 | 1 | 1 | 0 | 13 | 7.8 | 16.1785 | |
| Defects | A3 | 3 | 1 | 1 | 0 | 5 | 7.8 | 16.1785 | |
| Defects | A4 | 3 | 1 | 1 | 0 | 4 | 7.8 | 16.1785 | |
| Defects | A7 | 3 | 1 | 1 | 0 | 9 | 7.8 | 16.1785 | |
| Defects | Q1 | 3 | 1 | 1 | 0 | 15 | 7.8 | 16.1785 | |
| Defects | Q2 | 3 | 1 | 1 | 0 | 8 | 7.8 | 16.1785 | |
| Defects | Q3 | 3 | 1 | 1 | 0 | 9 | 7.8 | 16.1785 | |
| Defects | Q9 | 3 | 1 | 1 | 0 | 10 | 7.8 | 16.1785 | |
| Defects | Q4 | 3 | 1 | 1 | 0 | 8 | 7.8 | 16.1785 |
This data set contains one observation for each subgroup sample. The variables _SUBC_ and _SUBN_ contain the number of nonconformities 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 17.16:
title 'c Chart for Paint Defects in New Trucks'; proc shewhart table=Trucktab; cchart Defects*Truckid; label _SUBC_ = 'Number of Paint Defects'; 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.