Validation, calibration, revision and combination of prognostic survival models

The problem of assessing the validity and value of prognostic survival models presented in the literature for a particular population for which some data has been collected is discussed. Methods are sketched to perform validation through 'calibration', that is by embedding the literature model in a larger calibration model. This general approach is exemplified for x-year survival probabilities, Cox regression and general non-proportional hazards models. Some comments are made on basic structural changes to the model, described as 'revision'. Finally, general methods are discussed to combine models from different sources. The methods are illustrated with a model for non-Hodgkin's lymphoma validated on a Dutch data set.