XCHART Statement: SHEWHART Procedure

Example 17.37 Computing Process Capability Indices

You can save process capability indices in an OUTLIMITS= data set if you provide specification limits with the LSL= and USL= options. This is illustrated by the following statements: 

title 'Control Limits and Capability Indices';
proc shewhart data=Partgaps;
   xchart Partgap*Sample / outlimits = Gaplim2
                           usl       = 270
                           lsl       = 240
                           nochart;
run;

The data set Gaplim2 is listed in Output 17.37.1.

Output 17.37.1: Data Set Gaplim2 Containing Control Limit Information

Control Limits with Capability Indices for Gap Width Measurements

_VAR_ _SUBGRP_ _TYPE_ _LIMITN_ _ALPHA_ _SIGMAS_ _LCLX_ _MEAN_ _UCLX_ _LCLR_ _R_ _UCLR_ _STDDEV_ _LSL_ _USL_ _CP_ _CPL_ _CPU_ _CPK_
Partgap Sample ESTIMATE 5 .002699796 3 242.087 259.667 277.246 0 30.4762 64.4419 13.1028 240 270 0.38160 0.50032 0.26288 0.26288 




            

            


            
The variables _CP_, _CPL_, _CPU_, and _CPK_ contain the process capability indices. It is reasonable to compute capability indices in this case, since Figure 17.97 indicates that the process is in statistical control. For more information, see the section OUTLIMITS= Data Set.