The purpose of a predictive model is to provide valid outcome predictions for new patients. Essentially, the data set to develop a model is not of interest other than to learn for the future. Validation hence is an important aspect of the process of predictive modelling. An important distinction is between internal and external validation. We discuss internal and external validation techniques in this chapter, with illustrations in case studies.
Steyerberg, E. W. (2009). Clinical Prediction Models: Application of Prediction Models. Springer Science + Business Media, 101–111. https://doi.org/10.1007/978-0-387-77244-8