This question came from a webinar viewer:
“You said that a minimum of 20 data points is a good place to start in order to have a realistic view of the process.
What I was wondering is: Do you re-evaluate the average everytime a new data enters the lot or do you set a window of time which will serve as a sort of benchmark for the rest?”
Once you’ve established the baseline, you should NOT continually recalculate the average and limits. You should really only do so when there’s been s shift in the system (like a “Rule 2” signal with eight or more consecutive data points above or below the baseline average).
An exception to this might be if I create a PBC with just six data points to start. I might continue revising the average and limits until I get to 15 data points or so, but then I’d lock that in as the baseline.