On the meaning of the special-cause variation concept used in the quality discourse – And its link to unforeseen and surprising events in risk management

In quality management, 'common-cause variation' and 'special-cause variation' are key concepts used to control and improve different types of processes. In this paper we study the meaning of these concepts, having a special focus on the latter concept: how is the special-cause concept linked to ideas and concepts used in risk and uncertainty management, to reflect unforeseen and surprising events? In the quality discourse it is common to refer to two possible mistakes when confronting the variation: (i) to react to an outcome as if it were from a special cause, when actually it came from common causes of variation; and (ii) to treat an outcome as if it were from common causes of variation, when actually it came from a special cause. However, at the point of decision making it is difficult or impossible to know what the "true" state is. It is also appropriate to ask whether such a true state does in fact exist. In the paper we discuss these issues, the main aim of the paper being to improve our understanding of some of the fundamental concepts used in risk and quality management