"Normal" Credit: Mo
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Many people have been taught that capability indexes only apply to “normally distributed data.” This article will consider the various components of this idea to shed some light on what has, all too often, been based on superstition.
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Capability indexes are statistics
Capability and performance indexes are arithmetic functions of the data. They are no different from an average or a range, just slightly more complex. The four basic indexes are the following:
The capability ratio, Cp, is an index number that compares the [space available within the specifications] with the [generic space required for any predictable process].
The performance ratio, Pp, is another index number that compares the [space available within specifications] with the [estimated space used by process in the past].
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Process Capability
The basic question: How likely are you to drop the next part close to or beyond the specification? It really helps to plot the dots and look at the distribution, not just do the math.
Does Capability Require Normality?
Hi Don, I thought Cpk was describing process behaviour in the Control Chart) then placed in the Historgram with Specifications and the ratio to the closest Specirfication. AIAG SPC text may describe it better "It takes the process location as well as the capability into account. For bilateral tolerances Cpk will always be less than or equal to Cp. Cpk and Cp should always be evaluated and analyzed together. A Cp value significantly greater than the corresponding Cpk indicates an opportunity for improvement by centering the process." Is that same as what you said? "The centered capability ratio, Cpk, is an index number that uses twice the DNS to define the [effective space available within specifications] and then compares this with the [generic space required for any predictable process]."
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