The mean and standard deviation are used to calculate the center line and control limits. You can have Minitab estimate these parameters from the data or you can enter historical values. If you enter only one historical value, Minitab estimates the other parameter from the data.
If some subgroups have erratic data because of special causes that you have already corrected, you can omit these subgroups from the calculations to avoid incorrect parameter estimates.
In Omit the following subgroups when estimating parameters (optional), enter the subgroups. Use a colon to indicate a range of subgroups. Leave a space between each subgroup or range of subgroups. For example, to specify subgroups 4, 7, 11, 12, 13, and 14, enter 4 7 11:14.
By default, Minitab calculates the control limits using the actual subgroup sizes. When the subgroup sizes differ, the control limits are uneven, but you can force the control limits to be straight. Select Using the following size for all subgroups, 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.
When you specify a subgroup size, you change the calculations for the control limits, which can change the results of the tests for special causes. Use this option only if the differences between the subgroup sizes are small. Don't use this option when the difference between subgroup sizes is more than 25%. For example, if the largest subgroup has 10 observations and the smallest subgroup has 8 observations, then the difference is 20% ((10 – 8) / 10 = 0.2 = 20%).