C Chart - Options - Parameters
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Stat > Control Charts > Attributes Charts > C > C Chart Options > Parameters

You can enter historical data for estimating c. For example, if you have a known process parameter or estimates obtained from past data, you can enter the values. If you do not specify a value for the mean, Minitab estimates it from the data. You can omit or include certain subgroups when estimating parameters using Estimate.

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Mean: Enter the historical means as constants. If you enter one constant, all stages use that value; otherwise, enter a value for each stage.