Which chart is right?

The charts discussed in this section require the assumption of independence. 

Independence means that observations on any time period are based on the events in that time period and not on events in prior time periods.  For example, different patients rating of satisfaction in different time periods may be considered independent observations.  For another example, mortality from an infectious disease may not be considered independent because prior prevalence of infection has a strong influence on current rates of mortality.  Often independence is not met but assumed as a first approximation.

This page was last revised on 05/03/2015 by Farrokh Alemi, Ph.D.  This guide to control charts is part of the course on Quality / Process Improvement.