Editing Control chart (section)

===Calculation of standard deviation===

As for the calculation of control limits, the [[standard deviation]] (error) required is that of the [[Common- and special-causes|common-cause]] variation in the process. Hence, the usual [[estimator]], in terms of sample variance, is not used as this estimates the total squared-error loss from both [[common- and special-causes]] of variation.

An alternative method is to use the relationship between the [[range (statistics)|range]] of a sample and its [[standard deviation]] derived by [[Leonard Henry Caleb Tippett|Leonard H. C. Tippett]], as an estimator which tends to be less influenced by the extreme observations which typify [[Common- and special-causes|special-cause]]s.{{Citation needed|reason=What is the relationship? Ratio? How is it used? There's no way to look it up.|date=March 2019}}