iSixSigma

sigma value

Six Sigma – iSixSigma Forums Old Forums General sigma value

Viewing 9 posts - 1 through 9 (of 9 total)
  • Author
    Posts
  • #51109

    Quainoo
    Member

    Hello,
    If a process has a cpk value of 0,66 (2/3) and a cp of 1 (3/3) implying that the process is not centered, is it correct to say that the sigma level is 3 ? or is it 2 ?
    Thanks
    Vincent
     
     

    0
    #176651

    Craig
    Participant

    how about saying your cpk is 0.66?
     

    0
    #176658

    Nik
    Participant

    There are various rules of thumb for equating Cp & Cpk to Sigma scores, such as 1.33 Cp = 4 Sigma(short therm). If you can calculate Cpk, then you can calculate the Z score (assuming normal distribtuion). Cpk is the minimum of [(USL – X(bar))/(3*StDev)] and (X(bar)-LSL)/(3*StDev).
    If you have all the details to fill those formulas out, you can calculate the % yield using the Z distribution and then convert that to a Z-score. If you have MINITAB go Calc>Probability Distributions>Normal Distribution. Put your average and standard deviation in, do a cumulative probabilty, and put your LSL in for the constant. Write down the result. Do the same for the USL. Subtract the two (this is your % Yield). Then go back to Calc>Probability Distributions>Normal with the mean of 0 and standard deviation of 1. Put your % yield (calculated before) in the constant field. This is the Z score (sigma level).

    0
    #176659

    Quainoo
    Member

    Nik,
    Thanks very much for this detailed answer.
    If I go back to my example (cpk=0,66) , the Z score is 2 and therefore the sigma level is 2. Is that right ?
    Thanks
    Vincent
     

    0
    #176661

    Nik
    Participant

    Yes.

    0
    #176674

    Dr Peter Thompson
    Participant

    “Sigma levels” are totally meaningless.
    DO NOT USE THEM.
    3.4 dpmo is also totally meaningless.  There are plenty of papers published on how M Harry derived this rubbish.
     

    0
    #176683

    Jeff
    Participant

    Dr. Peter,So what metrics do you use?

    0
    #176687

    Dr. Phil
    Participant

    Jeff,
    Go do a little homework.
    Cp, Cpk, Pp, Ppk, Cpm all predated the “sigma” metric. They all do as good or better in describing the behavior of a process.

    0
    #176703

    Anup Kumar
    Participant

    Yes. This is right. Basically if the value is above 1 it is considered to be a good process. IF the value is 2, then it is a six sigma process.

    0
Viewing 9 posts - 1 through 9 (of 9 total)

The forum ‘General’ is closed to new topics and replies.