Saving Control Limits

CCHART Statement

Saving Control Limits

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.

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

Figure 33.3: The Data Set DEFLIM Containing Control Limit Information

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 $\sigma$ 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 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.

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

Figure 33.4: The Data Set TRUCKTAB

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;

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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