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Thread: control charts-type I error

  1. #1
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    control charts-type I error

    My control chart has a type I error rate of 0.0027 (3 standard deviations). I am comparing 5 months of signals to my baseline UCL. Am I inflating this error rate because I am doing multiple comparisons to the same UCL? The literature sends mixed signals. Some don't feel that a control chart is a true hypothesis test, while others would calculate the type I error to be 1-(0.9973)^5=0.013.
    Which is correct? Is the error inflated or not?

  2. #2
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    I don't consider control charts a good tool for hypothesis testing - they're meant to be a visual tool (that's why they're a chart...) to allow people to detect a process shift.

    To answer your question, I feel that 0.0027 is the probability, on a 3-sigma chart, of getting a point out of control over the long run, if the process is in control.

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