# Probability of points outside of the contorl limit

Six Sigma – iSixSigma Forums Old Forums General Probability of points outside of the contorl limit

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• #46108

Whitehurst
Participant

How can I estimate the probability of a next point in a control chart will fall outside of the control limits?

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#151948

Participant

Assuming your control limits are at +/- 3 sigma the probability of a point falling outside the control limits is 0.27% (since +/- 3 sigma covers 99.73% of your data set).

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#151949

Darth
Participant

Control limits are not probability limits.  Since data points are supposed to be independent, you can’t predict when the next data point will fall outside a control limit.  Once it does, you know the probability that it could have happenend by chance.

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#151952

Whitehurst
Participant

Thanks for your response .I agree

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