iSixSigma

Zst

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  • #48295

    BeRich
    Participant

    If a spec limit is 4.5 for zst and you get 1.6; isn’t this good news?  From what I understand your zst is the number of standard deviations you are from the mean. Is this correct? The way I see it is 1.6 is a tighter variance than 4.5. Also, is it correct to set up a lsl of 4.5 and an upper spec limit of 4.5?
     
     

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    #162308

    Mikel
    Member

    What in the world are you talking about?

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    #162323

    BeRich
    Participant

    Is 1.6 a good zst score?

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    #162324

    Mikel
    Member

    No – it’s awful

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    #162327

    BeRich
    Participant

    If a Z score indicates how many standard deviations a value, x , is from the mean then why is 4.5 better than 1.6?

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    #162330

    Darth
    Participant

    Think about it for a minute!!!!  If z is the number of standard deviations that the SPEC is away from the mean then a z value of 1.6 means the spec is close to the mean and lots of stuff will be beyond the spec…that’s not a good thing.

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    #162347

    ZST?
    Member

    Like Stan, I have no clue what you mean by “zst” (there are various programs out there called zst, and the term is used in microwave engineering, but what do you mean by “zst” in the context of z-values and six sigma.
    If you look at the conversion of z-values into sigma scores, here is a table that may answer your question (scroll down for the conversion).
    https://www.isixsigma.com/library/content/zdistribution.asp
     
    If you mean “zstd” here is a technical definition:
    ZSTD is the infit mean-square fit statistic t standardized to approximate a theoretical “unit normal”, mean 0 and variance 1, distribution. ZSTD (standardized as a z-score) is used of a t-test result when either the t-test value has effectively infinite degrees of freedom (i.e., approximates a unit normal value) or the Student’s t-statistic distribution value has been adjusted to a unit normal value. The standardization is shown on RSA, p.100-101. When LOCAL=Y, then EMP is shown, indicating a local {0,1} standardization.  When LOCAL=LOG, then LOG is shown, and the natural logarithms of the mean-squares are reported. More exact values are shown in the Output Files.
    Ben Wright advises: “ZSTD is only useful to salvage non-significant MNSQ>1.5, when sample size is small or test length is short.”

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    #162350

    BeRich
    Participant

    I am referring to a Z score – ST (meaning short term)  – Zst.
    What would cause a score of 1.6 compared to 4.5?
     

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    #162351

    BeRich
    Participant

    I am referring to a Z score – ST (meaning short term)  – Zst.
    What would cause a score of 1.6 compared to 4.5?
     

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    #162362

    Darth
    Participant

    Draw a picture of a normal distribution. Put the 1.6 spec away from the mean and see how much is outside of spec aka to the right of the spec. Now draw the spec at 4.5 away from the mean. How much is out of spec now? Which has less out of spec? Which is better assuming that you don’t want to make out of spec.

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