The present disclosure relates to a controller for controlling a system, capable of presentation of a plurality of candidate propositions resulting in a response performance, in order to optimise an objective function of the system. The controller has a means for storing, according to candidate proposition, a representation of the response performance in actual use of respective propositions; means for assessing which candidate proposition is likely to result in the lowest expected regret after the next presentation on the basis of an understanding of the probability distribution of the response performance of all of the plurality of candidate propositions; where regret is a term used for the shortfall in response performance between always presenting a true best candidate proposition and using the candidate proposition actually presented.