Really undestanding Statistical Process Control (SPC)

1°) Shewhart's original rule of statistical process control is:


"Variation from common-cause systems should be left to chance, but special causes of variation should be identified and eliminated."

So SPC indicates when an action should be taken in a process, but it also indicates when NO action should be taken. An example is a person who would like to maintain a constant body weight and takes weight measurements weekly. A person who does not understand SPC concepts might start dieting every time his or her weight increased, or eat more every time his or her weight decreased. This type of action could be harmful and possibly generate even more variation in body weight. SPC would account for normal weight variation and better indicate when the person is in fact gaining or losing weight.

2°) The rule should not be misinterpreted as meaning that variation from common causes should be ignored. Rather, common-cause variation is explored "off-line." That is, we look for long-term process improvements to address common-cause variation.

3°) A controlled process isn't necessarily a sign of good management, nor is an out-of-control process necessarily producing non-conforming product.