1. A method performed by one or more data processing apparatuses, the method comprising: receiving information corresponding to a plurality of conversion paths that each end with a respective conversion and each include a set of user interactions that occurred prior to the respective conversion by a respective user, the information including data relating to user interactions with a content item in the plurality of conversion paths within a marketing channel, wherein the content item is associated with one or more marketing activities; determining a first attribution credit that is based on a number of the plurality of conversion paths within the marketing channel and on a number of user interactions with the content item at a particular position in the plurality of conversion paths within the marketing channel; determining a second attribution credit that is based on the number of the plurality of conversion paths within the marketing channel and on a number of user interactions with the content item at a different particular position in the plurality of conversion paths within the marketing channel; based on the first attribution credit and the second attribution credit, determining an attribution contrast ratio that quantifies an effect of the content item at the particular position in the plurality of conversion paths within the marketing channel to an effect of the content item at the different particular position in the plurality of conversion paths within the marketing channel; identifying an opportunity based on the determined attribution contrast ratio, wherein identifying the opportunity comprises identifying marketing activities that are undervalued, and wherein an undervalued marketing activity has an attribution contrast ratio greater than a predefined number; and responsive to determining that a marketing activity, in the one or more marketing activities, associated with the content item is undervalued: presenting a recommendation for the marketing activity based on the identified opportunity, wherein the recommendation includes a bid value for the marketing activity within the marketing channel.