Editing Control chart (section)

==Criticisms==

Several authors have criticised the control chart on the grounds that it violates the [[likelihood principle]].{{Citation needed|date=March 2010}} However, the principle is itself controversial and supporters of control charts further argue that, in general, it is impossible to specify a [[likelihood function]] for a process not in statistical control, especially where knowledge about the [[cause system]] of the process is weak.{{Citation needed|date=March 2010}}

Some authors have criticised the use of average run lengths (ARLs) for comparing control chart performance, because that average usually follows a [[geometric distribution]], which has high variability and difficulties.{{Citation needed|date=March 2010}}

Some authors have criticized that most control charts focus on numeric data. Nowadays, process data can be much more complex, e.g. non-Gaussian, mix numerical and categorical, or be missing-valued.<ref name="Deng2012" />