XCHART Statement: SHEWHART Procedure

Example 18.37 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 18.37.1.

Output 18.37.1: 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, because Figure 18.97 indicates that the process is in statistical control. For more information, see the section OUTLIMITS= Data Set.