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Probability of violating SPC rules by chance

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  • #52064

    lin
    Participant

    Has anyone ever seen a list of the probabilities that a normal system will violate any of the standard SPC rules by chance?  For instance, I know that if a system is stable and normally distributed and my control limits are based on +/- 3 standard deviations that I will have points outside of my control limit 0.27% of the time.  What is the probability that I would get 7 consecutive points going up or down? And how would the probability be affected if I changed the number of consecutive points to 5 as you can in some software packages now?

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    #182534

    jomo
    Participant

    Bill:  Look up Nelson’s rules and Western Electric rules.

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