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Bimodal Cpk correction

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  • #53112

    Brody
    Participant

    Is there a formula for correcting the Cpk value for a bimodal distribution to take into account for both histograms being capable?
    The problem that I have is both histograms have excellent capabilities, but due to the nature of the processing the data is bimodal.  Since it is bimodal the overall Cpk is poor.
     

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    #188081

    MBBinWI
    Participant

    sure your measuring the right variable?  If you have bimodal data  the liklihood is you have two different items being measured.

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    #188083

    Brody
    Participant

    This is process data and his all from the same data set.  The issue with being bimodal is that due to the processing (i.e using different tools, different production cells,..etc) can cause a bimodal distribution.  Since the customer only cares about the end product, I cannot separate the data into 2 groups because of the processing differences.

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    #188086

    MBBinWI
    Participant

    So you are measuring different things.  My original hyptothesis has been vindicated.

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    #188123

    Remi
    Participant

    Brody,
    if your supplier would supply you their components in a bimodal form would you be happy? Even if all products were within specs?
    At the least you would want to get them in two batches. In that case you could improve the process by taking into account the move of the Mu (=between batches variation). For each of the batches a standard Cp(k) calculation would be satisfying. BUT YOU MAY NOT COMBINE THOSE Cp(k) VALUES!!!!Later on you would want that supplier to remove the variation source that caused that move of the Mu.
    So talk with your customer and ask him how much it is worth to him that you improve in this way. Our customer was very happy when we could supply him the components in batches with smaller variation and he had no problems at all with a moving Mu because he could correct this with a simple resetting of  the Temperature of his oven setting as long as he knew the Mu of each batch. Later on we were even able to fixate the Mu and he was even happier.
    Remi

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    #188157

    MBBinWI
    Participant

    Remi:  Just a little advice – if you are looking to get a response to a specific individual who has posted in a message thread, it’s better to post your reply to their post.  Otherwise, they have to go out and look for the reply.

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    #188207

    Remi
    Participant

    Hai MBBinWI,
    thanks for the advice. I didn’t see that Oops until you mentioned it. You are right.
    Remi

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