# How to calculate Cpk & Ppk if the tolerance changes during the period?

A

Hello everybody, I'm new here, I hope you can help me.

Do anyone know how to calculate cpk and ppk if in the period of calculation, the spec has changes?
For example during cpk calculation in November, the tolerance change from 10-13 cm to 11-14 cm in the middle of the month.
Can I calculate one by one (before and after) and make average? #### Bev D

##### Heretical Statistician
Staff member
Super Moderator
Re: How to calculate Cpk & Ppk if the tolerance change during the period?

You can, but that doesn't mean you should.
The capability should be be the current capability for the period and be actionable. At this point I would recommend only calculating the capability using the current tolerance. To do otherwise would imply a process change when only the tolerance has changed...

A

Re: How to calculate Cpk & Ppk if the tolerance change during the period?

Thanks Bev D. I'm doing this in order to not lose the "capability of process" before the tolerance change. Is doing the average for cpk correct scientifically?
You know they may have different data, etc2...

Anyone who can share their experience?

Thanks all!

#### Bev D

##### Heretical Statistician
Staff member
Super Moderator
Re: How to calculate Cpk & Ppk if the tolerance change during the period?

It is not statistically or practically correct to average the indexes.
If you don't want to 'lose the capability' calculate the indexes for the two periods separately. Remember the process hasn't changed. The tolerances did.

#### bobdoering

Trusted Information Resource
Here is a key point: did the process or its control change to accommodate the new tolerance? If so, you need to start fresh at that point. If not, then using the old data applying the new tolerance to the old data would still be applicable to understand its historical capability.

As BevD says, averaging a global statistic of this type is really statistical gibberish...more so than the indices themselves to begin with.

#### Bev D

##### Heretical Statistician
Staff member
Super Moderator
I might also ask what your real question is? in other words, what question are you trying to answer with the capability index calculations? this will allow us to give a much more helpful answer.

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