Polyhedrika

  Statistical Process Control (SPC) is used to control a critical process output, acting on the factors affecting it, only when required. The purpose is to differentiate between a statistically significant change in the process, and common cause variation, because the actions required will be different. Process stability is a prerequisite for any further analysis. If a process is not stable, any conclusions you draw, can't be assumed to hold in the future. With SPC we check for process stability in the future. Control over-reaction A case of over-reaction is illustrated by the following example: Someone is shooting at a target, and based on the deviation of the impacts, he adjusts the gun site after each shot. The result will be an increase of the dispersion of the impacts; therefore, the adjustments will make the process worse. The correct way is, of course, to fire 5 or 6 shots without adjustments, and then decide if adjustment is required, based on the center of the impacts. ...

  Process variation analysis is essential in order to improve it. There is always variation. Sometimes we only have random variation such as in the case of dice throwing: https://polyhedrika.blogspot.com/2020/12/dice-throwing-exercise.html   Normally we have additional variation caused by special causes and these are the ones we should analyse and reduce first. Hourly Production  We are going to analyse the hourly production data collected during one month: Download this Excel file Trend Analysis.xlsx from OneDrive folder Process stability SPC charts allow us to check for stability: see if all variation is random. https://polyhedrika.blogspot.com/2020/12/spc-analysis.html We now apply an individuals SPC chart to our production data to check this: The green and red dots are a clear indication of lack of stability: there are special causes affecting production. Timestamp Data Collection When we collect process data either manually or automatically it is essential to collec...