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XRCHART Statement |

The process capability index * C_{p} * is computed as

If you do not specify both LSL and USL, the variable _CP_ is assigned a missing value.

The process capability index *CPL* is computed as

If you do not specify LSL, the variable _CPL_ is assigned a missing value.

The process capability index *CPU* is computed as

If you do not specify USL, the variable _CPU_ is assigned a missing value.

The process capability index * C_{ pk} * is computed as

If you specify only USL, the index *C*_{pk} is computed as

and if you specify only LSL, the index *C*_{ pk} is computed as

The process capability index * C_{pm} * is computed as

where *T* is the target value specified with the TARGET= option.

When a single specification limit (SL) and target are specified,
*C*_{pm} is computed as

You can also use the CAPABILITY procedure to compute a variety of capability indices. The SHEWHART procedure and the CAPABILITY procedure use the same formulas to calculate the indices, but they use different estimates for the process standard deviation .

- The SHEWHART procedure calculates from subgroup estimates of . For details, see the previous section, "Methods for Estimating the Standard Deviation."
- The CAPABILITY procedure calculates as the sample standard deviation of the entire sample. For details, see "Standard Deviation" .

Regardless of which method you use, you should verify that the process is in statistical control before interpreting the indices, and you should verify that the data are normally distributed. The CAPABILITY procedure provides a variety of statistical and graphical tests for checking normality.

Some references use different notation and names for
capability indices.
For example, the manual *Fundamental Statistical
Process Control: Reference Manual* (1991) uses the term
"process capability indices" for the indices listed
in this section, and it uses the term "process
performance indices" for the indices computed by the
CAPABILITY procedure.

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