Shewhart dispersion charts made easy for mild to moderately autocorrelated normally distributed data

The aim is to make it easier for users to implement dispersion monitoring plans when the data are autocorrelated. If the data follow a low-order ARMA model then the same threshold for the monitoring plan is applicable by revising the rational subgroup size. A robust plan for non-normal data is achieved in most cases by first transforming the data to an approximate normal distribution, and then applying the plan for normally distributed data. The dispersion plan is based on the robust regression estimator of the slope coefficient when regressing the ordered rational sub-group values against their “expected” in-control values.