What causes Ppk to be greater than Cpk when a basic capability analysis is run?

[Jon O]

Hello Everyone,

I know this have been a hot point of discussion in many threads here on the Cove however I am bringing the whole Cpk/Ppk thing again with a different question.

The question of the hour is this: What causes Ppk to be greater than Cpk when a basic capability analysis is run on a set of data? Generally speaking the Ppk will usually be lower than the Cpk, due to the overall sigma calculation vesus the R bar/d2 calculation however on occasion we will have data that shows a higher Ppk.

Does this have to do with the subgroup variation?

Any insight would be appreciated.

Regards,

Jon

[KenK]

I suppose this could be related to the fact that Ppk's variance is estimated using the sample standard deviation and Cpk's variance is estimated using the range. I haven't taken the time to verify that though. My guess is that when you see Ppk larger, it isn't by much.

Ken

Sigma from Ppk is calculated using all of the individual measurements.

Sigma for Cpk is calculated using the variation of the subgroup totals. RbarD2

Since there is more variation in the individual measurements the subgroup totals, Ppk will usually be lower that Cpk.

That is why Ppk is a preffered methododology to use during initial capability analysis.

Generally I see <10% difference between the two.