A reader asked:
“I am wondering at what point do you recommend recalculating the average and upper and lower limits?”
Generally, we’d recalculate limits when there’s a signal that performance has shifted to fluctuate around a new average (evidence would be the “Rule 2″ signal of eight consecutive data points above or below the old baseline average.”
“I work in health care and have developed Process behaviour charts for many of our processes using the last 2 years of data. Do I leave the average calculation in my excel spreadsheet based on those initial 24 points? Or adjust the formula each month as I add data?”
No, it’s not a good practice to continually recalcuate each time there’s a new data point. We should only recalculate when there’s a signal / shift in performance. The average and the limits are established from a baseline timeframe of data points. If that baseline timeframe is a predictable system (meaning we have no signals in the chart), then we use the PBC to see if that predictable system is continuing or if it has changed.