COMMENTS ON TRADITIONAL AND 6 SIGMA CAPABILITY

Whereas we defined the short- and long- term capability in the traditional C _p and C _pk context, we are aware that in the 6-sigma (classical sense), the terms are reversed . In other words, the C _p and C _pk are for potential capability (short term), and the P _p and P _pk are for the ongoing capability (long term). This is an important distinction not so much in calculation but in the interpretation of the results. Also, the notation is different. For example,

• C _p = maximum allowable range of characteristics/(normal variation of process) - short term.

• C _pk = C _p (1 - k ), where k is the proportion of the tolerance zone consumed by the static mean shift.

  

  where T = target and ¼ = mean.

• P _p = maximum allowable range of characteristics/(normal variation of process) - long term.

• P _pk = P _p (1 - k ), where k is the proportion of the tolerance zone consumed by the static mean shift.

where T = target and ¼ = mean.

Special note:  

This notation and interpretation is not generally accepted by many quality professionals. However, it is what Harry (1997, pp. 8.16 “8.19) has introduced as part of the six-sigma methodology.

To the practitioner, it really does not make any difference what is used as long as it is consistent and one knows that there are really two capabilities to be content with.