1. A method of determining a bid modifier value to maximize a return on investment in a hybrid campaign comprising: determining, for one or more sets of keywords used in the campaign, a predicted conversion rate for a first device type and a predicted conversion rate for a second device type; calculating, by a processor, predicted campaign-level conversion rates for the first and second device types using the predicted conversion rates for the one or more sets of keywords used in the campaign; determining a ratio between the predicted campaign-level conversion rate for the first device type and the predicted campaign-level conversion rate for the second device type; and using the ratio as the bid modifier value that relates bids for the first device type to bids for the second device type.