1. A method of attributing credit for conversions to a plurality of event types, comprising the steps of: (a) defining a plurality of probabilities that a single impression of each of a plurality of respective event types causes a conversion by one of a plurality of users; (b) using a processor to select values for the plurality of probabilities so as to minimize a total squared error based on differences between an observed outcome for each user and a respective expected outcome for each user calculated from the plurality of probabilities.