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SPC - normal normal distribution?

Discussion in 'SPC - Statistical Process Control' started by essegn, Mar 9, 2019.

  1. essegn

    essegn Member

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  2. Bev D

    Bev D Moderator Staff Member

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    Control charts do not require a Normal distribution. Subgroup averages do not need to form a Normal distribution; the Central limit theorem does not apply.
    Anyone who teaches this is not knowledgeable about SPC.

    If you really want to understand SPC, read the works of Donald Wheeler, W. Edwards Deming, Davis Balestracci, Walter Shewhart (the creator of SPC). You can find Donald adn Davis’s works at quality digest for free.
     
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  3. Golfman25

    Golfman25 Well-Known Member

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    Maybe there was a misunderstanding between spc and cpk.
     
  4. essegn

    essegn Member

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    there is no need for a normal distribution, in order to calculate a Cpk.
    The normality is required the only for predicted (expected) ppm-Rate.
    Is this true?

    If so, what kind of misunderstandig between cpk and spc do you mean?
     
  5. Golfman25

    Golfman25 Well-Known Member

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    My understanding is traditional CPK formulas assume a normal distribution. So if your data isn't normally distributed you may need a different analysis.
     
  6. Bev D

    Bev D Moderator Staff Member

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    essegn is correct. As originally implemented Cpk did not take the shape of the distribution into effect. It was a simple, rough ratio of the process spread in relation to the tolerance range. In that context, the index had some informative value. Later pseudo statisticians ruined the index by trying to ‘predict’ the process defect rate. THAT does require knowledge of the actual distribution AND suffers from the gap between the theoretical infinite tails and the reality that real processes do not have infinite tails.
     

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