Excel formula for Ppk needed

J

jrubio

#11
Short term Vs Long Term

In my oppinion it exist two possibility to meassure the capability (Long Vs Short term).

Preliminary Study (100 parts)

1) 5 parts -> Pp1, Ppk1 (Normal Distribution)
2) 5 parts -> Pp2, Ppk2 (Normal Distribution)
3) 5 parts -> Pp3, Ppk3 (Normal Distribution)


....

20) 5 parts -> Pp4, Ppk4 (Normal Distribution)


you have 20 pp, ppk therefore you can map a potential preliminary curve haw well is improving your process. the curve tend to be exponential


pp
- -----------------------------------
- -
- -
- -
- -
- -
- - -1-2-3-4-5-6-7-8-9-10-11-12-13-14-14-16----20 (Size)


At this stage the short term ppk, pp is ppk20,pp20 but if you want a long term indez which take into acount all the 100 points as the curve is noraml by the Limit theorema you can add normal distribution and you will have other normal distribution therefore you can calcaulate the pp,ppk with all the 100 part to calculate the pp,ppk aggregated index.

----------------------

You see that the curve tend to a well pp, ppk (Lets say 3) now you can start with Cp, Cpk

1) 5 parts -> cp1, cpk1 (Normal Distribution)
2) 5 parts -> cp2, cpk2 (Normal Distribution)
3) 5 parts -> cp3, cpk3 (Normal Distribution)


....

20) 5 parts -> cp4, cpk4 (Normal Distribution)
 
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Tim Folkerts

Super Moderator
#12
How much detail do you need?

Basically:
1) enter the all the data
2) calculate the AVERAGE and STDEV for the data
3) Check which is smaller: (USL-AVERAGE)/(3*STDEV) or (AVERAGE-LSL)/(3*STDEV)

Is there something more specific you need help with? The actual use of Excel? The equations for Ppk?


Tim F
 
J

jrubio

#13
Tim Folkerts said:
How much detail do you need?

Basically:
1) enter the all the data
2) calculate the AVERAGE and STDEV for the data
3) Check which is smaller: (USL-AVERAGE)/(3*STDEV) or (AVERAGE-LSL)/(3*STDEV)

Is there something more specific you need help with? The actual use of Excel? The equations for Ppk?


Tim F

Yes it is plus Histogram
 

Jim Wynne

Staff member
Admin
#15
jrubio said:
Yes it is plus Histogram
Customer requirements aside, histograms can be useful when there are very large amounts of data, but when the number of individuals is relatively small, and you have the basic statistics at hand, a histogram isn't of much practical use.
 
J

jrubio

#16
Yes but all this Formulas have no sense if the Distribution is not normal. 100 points is not a low number of points is enough, Ford recomend minimum 3 to 5to obtain the sigma and average and therefore the Histogram.



The Histogram according to my criteria is mandatory.
 

Jim Wynne

Staff member
Admin
#17
jrubio said:
Yes but all this Formulas have no sense if the Distribution is not normal. 100 points is not a low number of points is enough, Ford recomend minimum 3 to 5to obtain the sigma and average and therefore the Histogram.



The Histogram according to my criteria is mandatory.
I don't know whether you're referring to Ford's requirements or your own personal requirements. You'll note that I said, "Customer requirements aside..." meaning that if you have a customer requirement for histograms, then you give them histograms, whether it makes sense or not.

If, on the other hand, you are referring in bold red letters to some personal requirement, I can only say that given 100 data points and the basic statistics, I don't need for someone to draw me a picture. If you do, that's fine. BTW, if you actually do need to see a chart, wouldn't an individuals chart be more useful, especially with regard to time?
 
J

jrubio

#18
why cp and not ppm?

Because at the end...

Isn´t it the purpose to obtain ppms. If you not have the Gauss how are you going to calculate?

To be honest I do not understand why Customer ask for Cp, Cpk, Pp, PPk instead of directly in ppm, which must be the useful data for them.

and ppm not depend of the curve, it is always the same ppm.

If someone know please the explanation must be much appreciated.
 
J

jrubio

#19
Jim Wynne said:
I don't know whether you're referring to Ford's requirements or your own personal requirements. You'll note that I said, "Customer requirements aside..." meaning that if you have a customer requirement for histograms, then you give them histograms, whether it makes sense or not.

If, on the other hand, you are referring in bold red letters to some personal requirement, I can only say that given 100 data points and the basic statistics, I don't need for someone to draw me a picture. If you do, that's fine. BTW, if you actually do need to see a chart, wouldn't an individuals chart be more useful, especially with regard to time?
------------
Great. I agree.

Great and logical exposition.
 

Jim Wynne

Staff member
Admin
#20
Re: why cp and not ppm?

jrubio said:
Because at the end...

Isn´t it the purpose to obtain ppms. If you not have the Gauss how are you going to calculate?

To be honest I do not understand why Customer ask for Cp, Cpk, Pp, PPk instead of directly in ppm, which must be the useful data for them.

and ppm not depend of the curve, it is always the same ppm.

If someone know please the explanation must be much appreciated.
Statistics such as Cpk are used as tools for prediction. PPM allegedly tells you how many things would be bad if you were to make a million of them, but it really has no predictive value, and makes no sense at all unless you are going to make a million of something. So Cpk and PPM are not directly comparable.

And this thread is straying :topic: .
 
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