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Difference between Cpm and Cm

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  • #51897

    Saherngu
    Participant

    Have a good day,
    I want do knoww diffrerence between Cpm index (SPC manual page 179) and Cm index . Could you help me? Thanks.
    Buen día,
    Necesito saber la diferencia entre los indicadores Cpm (Página 179 del manual SPC) y Cm. ¿Alguno me podría ayudar? Gracias.

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    #181615

    Gary Cone
    Participant

    Cm is not a very common metric, but as I understand it, it is the same
    as Cpk (I did a web search and found it in a book called Productive
    Objects by Robert Muller).Cpm is also sometimes referred to as the Taguchi capability index as
    it measures capability around a desired target value and goes to the
    same philosophy as Taguchi’s Loss Function.The calculation is the same as for Cp except target is substituted for
    mean in the calculation of standard deviation.

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    #181617

    Saherngu
    Participant

    Gary, Thanks a lot for your help.
    James

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    #181620

    Mike Carnell
    Participant

    James,
    I am not sure how much you want to pursue this but when Gary and I were at Motorola you could find some stuff on it (this goes back a while) as Cpt (capability to a target).
     

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    #181622

    Mario Perez-Wilson
    Participant

    And when I was at Motorola with Mike Carnell and Gary Cone we were using the capability ratio (CR)=6 x sigma /tolerance, where the tolerance =uls-lsl. Inverse to the Cp. LOLMario Perez-Wilson

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    #181623

    Bower Chiel
    Participant

    Hi James
    You will find the formula for Cpm in that wonderful free resource the NIST SEMATECH Engineering Statistics handbook – see http://www.itl.nist.gov/div898/handbook/pmc/section1/pmc16.htm. There is a good discussion of capability indices in Thomas Ryan’s book Statistical Methods for Quality Improvement. Note that if the process is on target then Cpm is the same as Cp. Ryan states the index is “both similar and superior to Cp since it reflects the extent to which the process mean differs from the target value for the mean”. He refers to Process 1 with USL=110, LSL=70, sigma=5,T=80,mu=75 and Process 2 with USL=60,LSL=20,sigma=5,T=40,mu=45. Both have Cpm of 0.94 but with normal distributions the non-conformance rates would be 16% and 0.1%respectively. Thus he concludes by pointing out that although Cpm is an improvement over Cpk it does not necessarily reflect process performance.I’ve not come across Cm.Best WishesBower Chiel

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    #181624

    Mario Perez-Wilson
    Participant

    James,I have seen the use of Cm and Cmk used in the context of Machine Capability Analysis applied to Surface Mount Technology chip-shooters or component placement. Where the formula for the Cm is identical to the Cp and the Cmk is identical to the Cpk, with the only difference is the application. Cm and Cmk is only for machine capability, and the Cp and Cpk for process capability.Mario Perez-Wilson

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