Abstract: Techniques for tuning model parameters to optimize online content are disclosed herein. In some embodiments, a computer system receives logged data for cohorts of users, where the logged data of each one of the plurality of cohorts comprises a number of impressions of online content to the cohort, parameter values applied to objective functions of a model used in selecting the online content for the impressions, contribution actions by the cohort directed towards the online content, and clicks by the cohort directed towards the online content. The computer system, for each cohort, selects one of the parameter values for each objective function based on the logged data. The computer system then selects at least one content item for display to a target user based on the model using the parameter values corresponding to the cohort of the target user.