When drafting protocols for the use of economic evaluation alongside clinical trials, it is common to have to specify which type of economic evaluation is going to be carried out. Will it be a cost-benefit analysis (CBA), cost-effectiveness analysis (CEA) or a cost-utility analysis (CUA)? It is our contention that prior specification of the appropriate economic technique is not possible, in the majority of cases, until data on effectiveness and cost are actually available. In this letter, we aim to demonstrate the thinking behind our contention and to illustrate this with two case studies; one of a recent randomised trial, the other of a trial currently in progress.