An advertising attribution system determines an attribution value for a set of advertising modalities associated with a conversion event. The modalities each provided an advertisement to a user who performed the conversion event. A conversion value associated with each of a plurality of modality subsets is determined representing the value to the advertiser of providing advertisements by the modalities in each modality subset. Based on the conversion value of each modality subset, a marginal value for each modality is determined for the set of modalities associated with the conversion event.