# std deviation

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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%

0
#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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