Computing Capability Indices

PROC CAPABILITY and General Statements

Computing Capability Indices

See CAPPROC in the SAS/QC Sample Library

This example is a continuation of the previous example and shows how you can provide specification limits with a SPEC statement to request capability indices in addition to descriptive statistics.

   proc capability data=cans normaltest freq;
      spec lsl=11.95 target=12 usl=12.05;
      var weight;
   run;

The options LSL=, TARGET=, and USL= specify the lower specification limit, target value, and upper specification limit for the weights. These statements produce the output shown in Figure 1.2 in addition to the output shown in Figure 1.1.

The CAPABILITY Procedure

Variable: weight (Fluid Weight (ounces))

Specification Limits
Limit		Percent
Lower (LSL)	11.95000	% < LSL	7.00000
Target	12.00000	% Between	77.00000
Upper (USL)	12.05000	% > USL	16.00000

Process Capability Indices
Index	Value	95% Confidence Limits
Cp	0.354967	0.305565	0.404288
CPL	0.420991	0.332644	0.508117
CPU	0.288943	0.211699	0.365112
Cpk	0.288943	0.212210	0.365677
Cpm	0.348203	0.301472	0.398228

Frequency Counts
Value	Count	Percents
Value	Count	Cell	Cum
12	1	1.0	1.0
12	3	3.0	4.0
12	1	1.0	5.0
12	2	2.0	7.0
12	3	3.0	10.0
12	8	8.0	18.0
12	6	6.0	24.0
12	6	6.0	30.0
12	10	10.0	40.0
12	11	11.0	51.0
12	5	5.0	56.0
12	6	6.0	62.0
12	6	6.0	68.0
12	6	6.0	74.0
12	10	10.0	84.0
12	6	6.0	90.0
12	4	4.0	94.0
12	2	2.0	96.0
12	1	1.0	97.0
12	2	2.0	99.0
12	1	1.0	100.0

Figure 1.2: Capability Indices and Frequency Table

In Figure 1.2, the table labeled Specification Limits lists the specification limits and target value, together with the percents of observations outside and between the limits. The table labeled Process Capability Indices lists estimates for the standard process capability indices^* C_p, CPL, CPU, C_pk, and C_pm, along with 95% confidence limits. The index C_pm is not computed unless you specify a TARGET= value. See "Standard Capability Indices" for formulas used to compute the indices.

If you specify more than one variable in the VAR statement, you can provide corresponding specification limits and target values by specifying lists of values for the LSL=, USL=, and TARGET= options. As an alternative to the SPEC statement, you can read specification limits and target values from a data set specified with the SPEC= option in the PROC CAPABILITY statement. This is illustrated in Example 1.1.

The FREQ option in the PROC CAPABILITY statement requests the table labeled Frequency Counts in Figure 1.2.

Chapter Contents
Previous
Next
Top