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Pignatiello & Ramberg

_ T-LSL . _ = —-- and C

pti

3ct

plu

USL-T

3ct

This index can be used when the process target T is not necessarily centered between the specification limits. This index can also be used when there is only one specification, in which case, C_pt is either C_pll or C_ptu.

The C_p index has been criticized sińce it does not consider the process' location or mean. Thus, C_p is often referred to as a measure of a

process' potential capability. To incorporate some measure of process location other indices have been proposed. One such popular index of process capability

C_pk:

r -miJ^USL-'^L

= mini

I 3a

3ct

where p is the process mean. The C_pk index is a ratio of the distance of the

process' mean from the nearest specification to half the process' natural tolerance spread. Thus it is defined for one-sided specifications, whereas C_p is

not.

An index (Bigelow 1992) that accommodates an explicit target and the process mean is C^,:

C_ph =min{C_pu'C_pklu}

where

Cphu ~

USL-T 3 CT

\^T-A

(USL-T)

and C_pkń =

\t-v\

3ct (T-LSL)

The index simultaneously considers the distance between a specification limit and the explicit target, and the distance between the process mean and the explicit target. For one-sided specifications, only C_pktu or is reported.

Finally, we will consider C^, still another measure of process capability that includes both location and spread. C,^ The index is defined as: