Select h and k from ARL (Average Run Length) tables. See Lucas^{5} and Lucas et al.^{6}
When the subgroup sizes differ, the control limits or V-mask are uneven, but you can force them to be straight. The following control charts display the same data.
By default, Minitab calculates the control limits or V-mask using the actual subgroup sizes.
Under When subgroup sizes are unequal, calculate control limits/V-mask, select Assuming all subgroups have size, and enter a subgroup size. This option is especially useful when all subgroups were intended to be the same size, but some subgroups are a different size. For example, some subgroups are smaller because of missed measurements. In that case, set the subgroup size to the intended size.