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not-Normal data – capab. analysis

Six Sigma – iSixSigma Forums Old Forums General not-Normal data – capab. analysis

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  • #41049

    Carl
    Participant

    Hello!  I am a greenbelt and planning a course next year for BB. 
    My boss asked me to measure our data. So,  I just completed 4-weeks of mesurement (All data) for our warehousing facility (cycle time of inventory).  I stacked the cycle time and got strange results.
    Process was not normal, but Control chart shows a stable process. Then, I used “Goodness of Fit test” and found  2-parameter exponential and Smallest extreme value as best fit.  I did the capability analysis (Capability / not-Normal) for both options, just to compare, and got the following results.
     
    1) 2-parameter exponential – skewed to the left and showed a z-bench of -0.83; 
    2) Smallest extreme value – strange histogram and z-bench of -0.2?
     
    I do not understand the capability charts at all. Did I do something wrong? Can someone please help me understand : a) how to interpret the results and explain to my Boss b) what to do next? Can I analyze root causes?
     
    Thank you.
     

    Regards,
    Carl

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    #128343

    LJLF
    Participant

    Try calculating the Cpk instead with a non normal distribution, such as Weibull.

    0
    #128367

    Carl
    Participant

    LJLF,
    Thanks for your inputs. 
    Does it mean I did it (cap. analysis calculation) wrongly? Why should I calculate using Cpk instead? Then, I won’t get a sigma level? Why did I get a skewed diagram and a negative sigma?
    Rgds,
    Carl

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