1. A computer-implemented method for evaluating a click fraud detection system configured to detect click fraud relating to online advertisements, the click fraud detection system being configured to classify each of a plurality of events as a valid click event or a fraudulent click event, at least some of the valid click events but none of the fraudulent click events being independently verifiable with reference to conversion data representing transactions associated with the valid click events, the click fraud detection system being susceptible to a first error type in which valid click events are classified as fraudulent, and a second error type in which fraudulent click events are classified as valid, the method comprising evaluating with a computing device operation of a first configuration of the click fraud detection system with reference to an objective function, the objective function having been derived by expressing a number of errors of the second error type in terms of a number of errors of the first error type with reference to the conversion data, and by assuming relative proportions of valid click events and fraudulent click events within the plurality of events, the objective function having been further derived with reference to a first cost associated with making errors of the first error type and a second cost associated with making errors of the second error type, and wherein the first and second costs are assumed to be substantially the same.