UCHART Statement: SHEWHART Procedure

Saving Control Limits

Note: See u Chart Examples in the SAS/QC Sample Library.

You can save the control limits for a u 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 defect counts from the data set Fabric (see Creating u Charts from Defect Count Data) and save the control limits displayed in Figure 17.87 in a data set named Fablim:

proc shewhart data=Fabric;
   uchart Defects*Roll / subgroupn = 30
                         outlimits = Fablim
                         nochart;
run;

The SUBGROUPN= option specifies the number of inspection units in each subgroup sample. The OUTLIMITS= option names the data set containing the control limits, and the NOCHART option suppresses the display of the chart. The data set Fablim is listed in Figure 17.88.

Figure 17.88: The Data Set Fablim Containing Control Limit Information

Control Limits Data Set FABLIM

_VAR_ _SUBGRP_ _TYPE_ _LIMITN_ _ALPHA_ _SIGMAS_ _LCLU_ _U_ _UCLU_
Defects Roll ESTIMATE 30 .002421390 3 .001671271 0.30333 0.60500



The data set Fablim contains one observation with the limits for process Defects. The variables _LCLU_ and _UCLU_ contain the lower and upper control limits, and the variable _U_ contains the central line. 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:

proc shewhart data=Fabric;
   uchart Defects*Roll / subgroupn = 30
                         outtable  = Fabtab
                         nochart;
run;

The data set Fabtab is listed in Figure 17.89.

Figure 17.89: The Data Set Fabtab

Number of Defects Per Square Meter and Control Limits

_VAR_ Roll _SIGMAS_ _LIMITN_ _SUBN_ _LCLU_ _SUBU_ _U_ _UCLU_ _EXLIM_
Defects 1 3 30 30 .001671271 0.40000 0.30333 0.60500  
Defects 2 3 30 30 .001671271 0.36667 0.30333 0.60500  
Defects 3 3 30 30 .001671271 0.30000 0.30333 0.60500  
Defects 4 3 30 30 .001671271 0.50000 0.30333 0.60500  
Defects 5 3 30 30 .001671271 0.23333 0.30333 0.60500  
Defects 6 3 30 30 .001671271 0.20000 0.30333 0.60500  
Defects 7 3 30 30 .001671271 0.16667 0.30333 0.60500  
Defects 8 3 30 30 .001671271 0.33333 0.30333 0.60500  
Defects 9 3 30 30 .001671271 0.26667 0.30333 0.60500  
Defects 10 3 30 30 .001671271 0.26667 0.30333 0.60500  
Defects 11 3 30 30 .001671271 0.46667 0.30333 0.60500  
Defects 12 3 30 30 .001671271 0.16667 0.30333 0.60500  
Defects 13 3 30 30 .001671271 0.30000 0.30333 0.60500  
Defects 14 3 30 30 .001671271 0.43333 0.30333 0.60500  
Defects 15 3 30 30 .001671271 0.23333 0.30333 0.60500  
Defects 16 3 30 30 .001671271 0.16667 0.30333 0.60500  
Defects 17 3 30 30 .001671271 0.26667 0.30333 0.60500  
Defects 18 3 30 30 .001671271 0.36667 0.30333 0.60500  
Defects 19 3 30 30 .001671271 0.23333 0.30333 0.60500  
Defects 20 3 30 30 .001671271 0.40000 0.30333 0.60500  



This data set contains one observation for each subgroup sample. The variables _SUBU_ and _SUBN_ contain the number of nonconformities per unit in each subgroup and the number of inspection units per subgroup. The variables _LCLU_ and _UCLU_ contain the lower and upper control limits, and the variable _U_ contains the central line. The variables _VAR_ and Roll 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 by the SHEWHART procedure. For example, the following statements read Fabtab and display a u chart (not shown here) identical to the chart in Figure 17.87:

title 'u Chart for Fabric Defects';
proc shewhart table=Fabtab;
   uchart Defects*Roll / subgroupn=30;
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.