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Quantile Cpk

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

  1. Pongsakorn

    Pongsakorn Active 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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    Pongsakorn, Oct 8, 2018
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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.
     
    Bev D, Oct 9, 2018
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    Pongsakorn likes this.
  3. Pongsakorn

    Pongsakorn Active Member

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    Bev, thanks for your advice and comment.
     
    Pongsakorn, Oct 30, 2018
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