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std deviation

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This topic contains 2 replies, has 3 voices, and was last updated by  Go USA 14 years, 5 months ago.

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  • #39430

    Scott
    Member

    For normal data, what % of the data will fall 1.3 std deviations from the mean.  I am confused on how to translate this to an exact number.  My guess is it is roughly 77%

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    #119879

    FTSBB
    Participant

    80.6399%
    Look at the probability areas covered by Z values of -1.3 and 1.3 on a standard normal curve, should be available in almost any quality book or this link:
    https://www.isixsigma.com/library/content/zdistribution.asp
    If you have Excel, use the following formula:
    = normsdist(1.3) – normsdist(-1.3) = 80.6399%

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    #119903

    Go USA
    Participant

    Hi Rick,
    Send me your email address and I will forward you a nice tool that converts standard deviation into percent defect rate. In Minitab you can also select Calc> Probability Distributions> Normal>
    In the dialog box

    Select Cumulative probabilities
    Select Input Constant
    Enter the distance your spec is from the mean in units of std dev (1.4)
    you will get the answer 91.92% yield
    (1-91.92%) = 8.08% defect rate
    Best Regards,
    TD

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