On the use of surrogate end points in randomized trials

In a recent paper Day and Duffy proposed a strategy for designing a randomized trial of different breast cancer screening schedules. Their strategy was based on the use of predictors of mortality determined by patients' factors at diagnosis as surrogates for true mortality. On the basis of the Prentice criterion for validity of a surrogate end point, and data from earlier studies of breast cancer case survival, they showed that, not only would the trial require a much shorter follow-up, but also that the information (i.e. inverse variance) for evaluating a treatment effect on mortality would be greater by a factor of nearly 3 if the predictors of mortality were used, compared with a trial in which mortality was actually observed. Although these results are technically correct, we believe that the conceptual strategy on which they are based is flawed, and that the fundamental problem is the Prentice criterion itself. In this paper the technical issues are discussed in detail, and an alternative structure for evaluating the validity of surrogate end points is proposed.