Fig. 2

A summary of methodological guidance for model validation. Internal validation is the minimum requirement for clinical prediction models. External validation is recommended to evaluate model generalizability in different but plausibly related settings. Designs for validation studies differ in strength (e.g., temporal validation is a weak form of validation, and data splitting is discouraged). Examination of two validation aspects (discrimination and calibration) is recommended for assessing statistical performance irrespective of the type of validation. Clinical usefulness is a common area between validation and impact assessment, and its examination is advised for assessing the clinical performance of models intended to be used for medical decision-making