Figure 2.

Number of studies published per year. Development=studies of which the main aim is to develop a novel prognostic model. This category may also include studies that, besides developing a model, also externally validate an existing model. External validation=studies that solely externally validate an already existing model. Update=studies of which the main aim is to update an existing model. This category may also include studies that, besides updating a model, also externally validate that same model.