A common cause is the set of factors that are continuously present in a process and make up its natural variation. It is also referred to as chance cause or random cause.
Small fluctuations in ambient temperature, variations in raw material within the permitted range, the natural vibration of the machine and normal differences between operators fall into this category.
Difference from special cause
Common causes are part of the process and do not produce a signal on the control chart; the control limits are calculated according to this variation in the first place. Special causes come from outside and produce a signal.
Improvement approach
Reducing common cause variation is possible not through intervention in individual events but through system change: equipment renewal, environmental control, tightening the raw material specification, strengthening standardized work.
This usually requires a management decision and investment; it cannot be solved at operator level.
If the common cause variation of the process is large relative to the tolerance width, the capability indices come out low, and this is not something that can be corrected by adjustment.
The distinction between common and special causes and variation reduction approaches are addressed in the SPC training. For structural improvement of the process, see the FMEA and Control Plan programmes.