Ewout Steyerberg is associate professor at the Department of Public Health, Erasmus MC, Rotterdam, the Netherlands. He has been involved in many clinical studies with emphasis on cost-effectiveness, prognostic factors and prediction.

Frank Harrell is professor at the Department of Health Evaluation Sciences, University of Virginia, Charlottesville, VA, USA. He is an expert in predictive modeling and recently published a book on the topic (‘Regression Modeling Strategies’, Springer, 2001).

Both have worked on predictive models for many years, and have recognized model validation as of paramount importance. It is well possible to develop a model with apparently provides adequate predictions, but fails when validated on new patient data.