1. A method performed by data processing apparatus, the method comprising: receiving information corresponding to a plurality of conversion paths, the information including data relating to user interactions with a content item, wherein the content item is associated with a marketing activity; determining a first attribution credit by applying a first attribution model to the received information, and a second attribution credit by applying a second attribution model to the received information; determining an attribution contrast ratio based on the first and second attribution credit; 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 1; and presenting a recommendation for the marketing activity based on the identified opportunity, wherein the recommendation includes a bid value for the marketing activity.