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Span Process Capability

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  • #30230

    Brian Kluchnick
    Participant

    I am trying to establish process capablity for span data (29 thousand elements, ranging from -351 to 1400).  My data is non-normal. Since I have negative values, I am unable to transform the data. 
    Does anyone now the best to deal with this type of situation and establish process capability?
    Thanks.

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    #78527

    Marc Richardson
    Participant

    Why can’t you just add 351 to all the values first and then try to tranform them? If that won’t work. then use nonparametric methods.
    Marc Richardson
    Sr. Q.A. Engineer

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    #78539

    Carl H
    Participant

    1.  I would add 352 (gets all values >0) to each value then do appropriate transformation to get it Normal. 
    2.  Run a capability analysis anyhow.  Read the obseved % or DPMO values out of spec limits.  Get Z value(s) from Z table. Example.  2.0% out of spec high, 1.0% out of spec low.  Z table gives Z = 1.88 at 3%. 
    3.  Try a probablity plot (using software).  Read off the observed % from this and look up Z again.
    Carl
     

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