Evaluation of performance
When we develop or validate a prediction model, we want to quantify how good the predictions from the model are (“model performance”). Predictions are absolute risks, which go beyond assessments of relative risks, such as regression coefficients, odds ratios, or hazard ratios. We can distinguish apparent, internally validated, and externally validated model performance (Chap. 5). For all types of validation, we need performance criteria in line with the research questions, and different perspectives can be chosen. We first take the perspective that we want to quantify how close our predictions are to the actual outcome. Next, more specific questions can be asked about calibration and discrimination properties of the model, which are especially relevant for prediction of binary outcomes in individual patients. We will illustrate the use of performance measures in the testicular cancer case study, with model development in 544 patients, internal validation with bootstrapping, and external validation with 273 patients from another centre.