Cpk vs PpK
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Quainoo.
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May 18, 2007 at 6:36 pm #47035
TierradentroParticipant@johnInclude @john in your post and this person will
be notified via email.What is the difference between Cpk vs Ppk?
0May 18, 2007 at 6:44 pm #156276Process Capability vs Process Performance
0May 18, 2007 at 7:36 pm #156281
TierradentroParticipant@johnInclude @john in your post and this person will
be notified via email.CT-do you have additional info on Cpk vs Ppk ? article, website, etc.
0May 18, 2007 at 8:11 pm #156284On the top right hand corner of this website is a search engine. type in Ppk and boom all the info you could ever want to know is right there. Same thing for Cpk. Google will get you about 3.34 million hits in .26 seconds also
0May 19, 2007 at 6:42 am #156291Short-term Vs. Long-term
In the calulation of SD : for Cpk (R-bar/d2),for Ppk you use the individual formula for calculating the SD.
Both are use for centering
good luck0May 19, 2007 at 8:49 am #156293Hi John
Ppk tells u the actual Process performance. It contains both Special Cause as well as Common Cause of variation (also known as Standard Deviation).
Cpk tell u the potential of the process when all the special cause are removed from the current process. It tell the process Potential when all the special cause of variation is removed. It only includes Common Cause of variation for calculation.
Thus Ppk tell the actual performance of the period of which the data is calculated while Cpk tell the potential that one can achive when all Special Cause of variation is removed from the process
the formulas u can refer from any book
Broadle
Total cause of variation= standard Deviation
Common Cause = SMR2 (generally) but could vary with grouping variable
Cheers
Gaurav
0May 19, 2007 at 9:08 am #156295
WaskitaParticipant@ferry-waskitaInclude @ferry-waskita in your post and this person will
be notified via email.John,
Visit the following link where you can find all about CpK, PpK
http://www.qualityadvisor.com/library/capability_FAQ.php#difference_between_Cp_and_Pp
Good luck!0May 20, 2007 at 11:11 am #156311Hi Gaurav,
One thing is not very clear for me.
I can’t see why, just by using the formula R/D2 (in order to calculate the sigma value), Minitab manages to go from long term to short term and thus “remove all special causes”. I can’t see the logic. Any idea ?
Thanks
Vincent
0May 21, 2007 at 1:12 pm #156342I would go so far as to say it removes “all” special causes. What it does is remove any fluctuations in the mean which may be due to special causes.
0May 22, 2007 at 5:39 am #156385Hi Vincent
I am not very sure of what Minitab uses. But I will give u the logic
For calculating Short Term standard deviation (SD) with-in batch SD is used. Now if you have 10 Batches and 5 samples each batch. The SD of each batch is considered for calculating Short term SD
SMR2 is generally taken if for every batch there is only 1 sample. Now the logic behind Short Term SD is somewhat similar to 5 point moving average. The special cause is suppressed when the moving average is taken. So if you plot a trend chart (Run Chart) and 5 or 8 point moving average of same data over it you can note the variation in two. The 5 or 8 point moving average will have less variation and special cause is averaged out by other data.
Though its is pretty difficult to separate out all special cause of variation but this it is suppressed by this method
Hope it helps
for more detail u can contact me on my email
[email protected]
Cheers
Gaurav
0May 26, 2007 at 9:19 am #156591Hi Gaurav, Hi Chris,
Many thanks for your answers (I have been away for the week)
Gaurav, thanks for your e-mail address, I am going to make some experiments and I may contact you in the near future. Thanks again.
Vincent
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