Interpreting Capability Indices of Normal Dist. Upper Boundary
September 17, 2010 at 8:52 pm #53578
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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.
Marty0September 18, 2010 at 5:22 am #190760
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