Stat > Control Charts > Attributes Charts > C > C Chart Options > Estimate
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.
From the drop-down list, specify whether you want to list the subgroups to omit or the subgroups to include.
To list the subgroups to omit, select Omit the following subgroups when estimating parameters (eg, 3 12:15).
To list the subgroups to include, select Use the following subgroups when estimating parameters (eg, 3 12:15).
List 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.