What is the formula I need to calculate Pp and Ppk?

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Bev D

Heretical Statistician
Staff member
Super Moderator
#13
garbage in garbage out.

not all distributions are Normal, not all sampling for process capability is correctly done, not all processes are shown to be stable prior to calculating capability, knowledge of the formulas and their intent IS important in interpreting the resutls.


or as Hubert Blalock said: "The manipulation of statistical formulas is no substitute for knowing what one is doing"
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#14
Re: Capability calculations need 30 to 100 samples for acceptable confidence band

I'm trying to sort out Cpk and Ppk too. So... if I am understanding this correctly, the calculations for Cpk and Ppk are identical.... just use a different sigma.

But that doesn't match my basic understanding ...Cpk is how well curve fits between the control limits and Ppk is how well the curve and limits line up with the specifications. How can the same calculation give two different things. :confused:
There are a lot of posts on this very topic here. I would suggest searching for them and then coming back for specific questions. : )
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#15
Garbage in - garbage out. It is just a number cruncher. If you do not have a thorough understanding of the source data, it can lead you down a thorny path.

For example: When you calculate a variance (and therefore a standard deviation) of a process, it really represents the total variance. Total variance is a composite of many variances, and they may be of differing distributions - normal, uniform, Weibull - all in differing levels of participation. Making valid decisions likely requires understanding these underlying distributions. The software will not know this, it is not that bright. The user has to ponder these things.

(Yikes...Bev beat me to it!)
 
R

Rebecca Bowes - 2010

#16
I am experiencing that now on the job....non-normal. We are going to take a larger sampling, we are thinking that we didn't get all of the population even though our initial sampling plan was thought to be good.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#17
I am experiencing that now on the job....non-normal. We are going to take a larger sampling, we are thinking that we didn't get all of the population even though our initial sampling plan was thought to be good.
Could be the distribution is truly non-normal! It could happen! Which means your entire population will be non-normal, so your sampling may have been correct.

What is the process?
 
R

Rebecca Bowes - 2010

#18
Blowmolding at this stage, then once that process is stable, the into maching/cutting, welding and then finally assembly. We need to get the blowmolding process stabalized as much as possible in the critical areas first. Yes, it maybe non-normal, but won't know until we actually measure more.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#20
Blowmolding at this stage, then once that process is stable, the into maching/cutting, welding and then finally assembly. We need to get the blowmolding process stabilized as much as possible in the critical areas first. Yes, it maybe non-normal, but won't know until we actually measure more.
I could see a blow molded dimension being non-normal - maybe a Weibull or such. Perhaps similar to a unilateral tolerance, but based on physical limitations (such as the part can be no larger than the mold, but it can shrink smaller...)

Just a thought...
 
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