iSixSigma

Cpk vs PpK

Viewing 11 posts - 1 through 11 (of 11 total)
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  • #47035

    Tierradentro
    Participant

    What is the difference between Cpk vs Ppk? 

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    #156276

    CT
    Participant

    Process Capability vs Process Performance
     

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    #156281

    Tierradentro
    Participant

    CT-do you have additional info on Cpk vs Ppk ? article, website, etc.

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    #156284

    CT
    Participant

    On 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

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    #156291

    Satan
    Member

    Short-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  luck

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    #156293

    Gaurav
    Participant

    Hi 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
      

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    #156295

    Waskita
    Participant

    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!

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    #156311

    Quainoo
    Member

    Hi 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
     

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    #156342

    walden
    Participant

    I 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.

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    #156385

    Gaurav
    Participant

    Hi 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
     

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    #156591

    Quainoo
    Member

    Hi 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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