Traditional sample size calculations for randomized clinical trials depend on somewhat arbitrarily chosen factors, such as Type I and II errors and the smallest clinically important difference. An effectiveness trial (otherwise known as a pragmatic trial or management trial) is essentially an effort to inform decision-making, i.e. should Treatment be adopted over Standard? As such, a decision theoretic approach will lead to optimal sample size determination. Using incremental net benefit and the theory of the expected value of information, and taking a societal perspective, one can determine the sample size that maximizes the difference between the cost of doing the trial and the value of the information gained from the results. In addition, expected value of information methods can be used to interpret trial results. The methods will be illustrated using an example.