Calculation of σ within subgroups for Cpk in Minitab

J

John Koh

#1
In Minitab, the std deviation used to calculate Cpk is the sigma for within subgroups whereas for Ppk, the overall sigma is used to calculate. My question is that since there are so many subgroups (for example 25 subgroups), which subgroups' sigma will be used to compute the final Cpk?
 
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Bev D

Heretical Statistician
Staff member
Super Moderator
#2
Most statistical software use the most common definition of Cpk adn Ppk:
Cpk is for short term capability and uses the within subgroup variation. it gives you an idea of the potential Capability if you were to eliminate the subgroup to subgroup (assignable cause) variation*.
Ppk is for long term capability and uses the total standard deviation - both within and between - and gives you an idea of the actual Performance.

in general use, many people simply refer to Cpk as the total performance capability (the Ppk formula in most software). by the way, this is the ORIGINAL definition of Cpk, before someone dreamed up the Ppk thing.

When you have a homogenous process - or have rationally subgrouped - Cpk and Ppk will converge to the same value.

you are probably looking for the Ppk Minitab output...


*this distinction is actually in direct opposition to the 'requirement' that processes be statistically in control before calculating Cpk/Ppk...but hey the whole Cpk/Ppk thing is statistically invalid so what the heck...
 
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