control charts-type I error

#1
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?
 

JohnM

TS Contributor
#2
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.