Original Source:  Jeff Heyl. © 2011 Pearson Education, Inc. publishing as Prentice Hall.

 “The objective of a process control system is to provide a statistical signal when assignable causes of variation are present. 

Variability is inherent in every process Natural or common causes Special or assignable causes Provides a statistical signal when assignable causes are present Detect and eliminate assignable causes of variation Points which might be emphasized include:

 – Statistical process control measures the performance of a process, it does not help to identify a particular specimen produced as being “good” or “bad,” in or out of tolerance.

– Statistical process control requires the collection and analysis of data – therefore it is not helpful when total production consists of a small number of units – While statistical process control can not help identify a “good” or “bad” unit, it can enable one to decide whether or not to accept an entire production lot. If a sample of a production lot contains more than a specified number of defective items, statistical process control can give us a basis for rejecting the entire lot. The issue of rejecting a lot which was actually good can be raised here, but is probably better left to later…”