We are strong believers in (Dr. Demings teachings about) businesses being a process. All processes have variation. The role of quality is to stabilize the system as much as possible and thus to reduce variation.
From D. Wheelers book "Understanding Variation" (SPC Press 2000) we took the structure and formulas to create this process analysis spreadsheet [76 KB] which helps separate special cause variation from common cause variation.
The only input for a fact based decision making process should be signals from an underlying special cause from within the process. Touching common cause variation means tampering with the system - it will make things worse.
Rules of thumb: a process may show special cause variation if
- a single observation is outside the process limits or
- 3 out of 4 consecutive observations are situated closer to one process limit than to the process average or
- 8 consecutive observations are situated on one side of the process average.
Then and only then should you make an intervention to the process.