A method performed by one or more computing devices. The method uses data identifying a plurality of users and a plurality of achievements. The data identifies which of the plurality of achievements each of the plurality of users has earned and when any such achievements were earned. The method includes ordering the plurality of achievements to define an ordered achievement list, and for each of the plurality of users, removing from the ordered achievement list any of the plurality of achievements identified in the data as having been earned by the user to define a recommended achievement list for the user. The method also includes identifying a selected achievement, and identifying as a set of similar users any of the plurality of users whose recommended achievement list includes the selected achievement.