External validation of a prognostic index

Background Prognostic indices, derived from prognostic models, can be used to stratify patients in clinical trials. This includes stratifying eligible patients for randomisation, stratifying randomisation, or identifying subgroups for the trial analysis. However, before an index can be used for these purposes, it needs to be validated externally in independent data. We demonstrate methods of external validation, including methods for handling a covariate missing from the validation dataset, via a prognostic model for risk of seizure recurrence following a first ever seizure.