1. A method of attributing credit for conversions to a plurality of event types, comprising the steps of: (a) defining a first plurality of probabilities that a single impression of each of a plurality of respective event types causes a conversion by one of a first subset of a plurality of users, each of whom is exposed to only one type of event; (b) defining a second plurality of probabilities that a single impression of each of the plurality of respective event types causes a conversion by one of a second subset of the plurality users, each of whom is exposed to at least two types of events, the second plurality of probabilities reflecting synergy between at least two of the event types; (c) tracking impressions from the first and second subset of plurality of users; (d) tracking, for the impressions, the event types and conversions by the plurality of users; and (e) using a programmed processor to calculate values for the first and the second plurality of probabilities based on the tracked impressions and conversions.