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Quantile Cpk

Discussion in 'Capability - Process, Machine, Gage …' started by Pongsakorn, Oct 8, 2018.

  1. Pongsakorn

    Pongsakorn Member

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    JESD557C para 5.3 defines the "Quantile Cpk" formula to be use for non-normal and and non-log-normal data but I still do not understand clearly, please advise as follows:
    - What is the different between "Cpk" and "Quantile Cpk"?
    - When to use "Quantile Cpk"

    upload_2018-10-8_17-55-39.png
     

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  2. Bev D

    Bev D Moderator Staff Member

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    The quantile approach is an attempt to align a non-normal distribution standard deviation to a defect rate estimation.
    it's theoretically OK. there are many approaches to deal with non-normal distributions - this is one of them. Quantiles is a non-parametric approach that is theoretically distribution-less. you simple divide the data into 4 segments with 25 of the data in each 'chunk'. the chunks will have varying lengths. (google 'box plot' it uses the same concept)
    If you have a Normal distribution you just calculate the Cpk in a straight forward manner.


    HOWEVER: this is all a bunch of hoo-ha. lots of math to no valuable end. it only serves to check some box for a Customer or internal QA person who doesn't understand variation. Cpk is a zombie theory - it just won't die. no matter how much logic and reason and science we throw at it. if you don't have to do it, don't. total waste of time.
     
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  3. Pongsakorn

    Pongsakorn Member

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    Bev, thanks for your advice and comment.
     

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