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Any deviation from the desired sigma level is termed as fail in case of typical normal distribution For example you are making nuts with root diameter 50” tolerance +/- 0.01”. A component is failed/rejected if it is less than 49.99” or greater than 50.01” In case of fatigue this is slightly different because, it is life number instead of diameter. If life required is 1000Cycles. You can express tolerance of +/- 5 cycles. However if the life turns out to be 1005 cycles it is to your advantage. Rejection is there only if the life is less than 995 cycles. Standard Deviation is considered only on the negative side to the mean in SN/EN curves, this is called “certainty of survival” in Fatigue 1 sigma Standard deviation( 68.27%) becomes 84.135% Certainty of survival in fatigue 2 sigma Standard deviation (95.45%) becomes 97.725% Certainty of survival in fatigue