Enter a single mean for the chart. The mean is used to calculate the center line and control limits.
Omit subgroups from the estimation of the mean
In Omit the following subgroups when estimating the mean (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.
Force control limits to be straight
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.
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%).
For example, the data for the following charts is the same, but the control limits for the second chart were calculated based on a fixed subgroup size.