?
Interpretation of Shewhart Control Charts: New Challenges and Opportunities
This work shows that an interpretation of signals on Shewhart control charts (SCC) can be different from commonly used as soon as one takes into account that an assignable cause of variation can change not only the parameters of the underlying distribution but its type as well. For example, the lack of points beyond 3σ level can mean not the problems of rational subgroup formation as it is usually being explained in literature but it may be caused by a change of the distribution function (DF). Or a point beyond the control limits may signal not about the change of average or standard deviation (SD) but about the DF's change while mean and SD may remain unchanged. As a result the significance of formal approach to SCC interpretation decreases and new opportunities for process analysis emerge. At the end of this paper we stated some problems that should be discussed by all interested in the development of Statistical Process Control (SPC) and statistical thinking.