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Do you have historical data on how many people, with certain characteristics, responded to emails previously? If yes, then you can train a model (eg ANN, CART) using topic / gender / domicile / employment status (and optionally size of group of these people) as input features and number of responses of that group of people (or percent of the original size) ...


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If you are evaluating point estimates (i.e. single number estimates) then you are well advised to use a proper scoring rule. Some metrics elicit "honest" estimates whereas others do not. A characterization of proper scoring rules was provided by Savage. Most people go with squared error, logarithmic. Contest sites like M-Competitions or ...


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