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Interpreting Capability Indices of Normal Dist. Upper Boundary

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  • #53578

    Doa
    Participant

    Hi To All,

    I have a dilemma when running a Minitab Capability Analysis

    A) For a Normal Dist., a process with LSL and USL and a target, Minitab only calculates Cpm, Cpk and Ppk, Cp, and Pp. The indices indicate process off target Cpm = 0.8, Cpk = 0.91 Ppk= 0.73 but relatively low variation Cp= 1.64 Pp= 1.32.

    Expected Overall Performance (defects all the upper side) =13,957

    B) For a Normal Distribution with LSL and a hard or upper boundary of 10.0. Only the LSL is left for Minitab to calculate indices; the Cpk and Ppk indices increase, Cpk = 2.36 , Ppk = 1.91 and Cpm = 1.61; and no Cp or Pp indices are calculated.

    Expected Overall Performance = 0.01.

    In both cases the mean is shifted slightly to the high side of the target of (9.16 vs. 9.0).

    My question, although everything appears to be in order, which is the correct expected overall perfromance? It would seem that although very low the second is the correct interpretation. Any insight?

    Many thanks for your assistance.

    Marty

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    #190760

    Rohatgi
    Participant

    When only LSL is mentioned.
    It means anything >LSL is acceptable (to customer).

    In other words, capability of your process is increased.
    That is what is shown in capibilty indices.

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