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A prognostic model should not enter clinical practice unless it has been demonstrated that it performs a useful role. External validation denotes evaluation of model performance in a sample independent of that used to develop the model. Unlike for logistic regression models, external validation of Cox models is sparsely treated in the literature. Successful validation of a model means achieving satisfactory discrimination and calibration (prediction accuracy) in the validation sample. Validating Cox models is not straightforward because event probabilities are estimated relative to an unspecified baseline function.We describe statistical approaches to external validation of a published Cox model according to the level of published information, specifically (1) the prognostic index only, (2) the prognostic index together with Kaplan-Meier curves for risk groups, and (3) the first two plus the baseline survival curve (the estimated survival function at the mean prognostic index across the sample). The most challenging task, requiring level 3 information, is assessing calibration, for which we suggest a method of approximating the baseline survival function.We apply the methods to two comparable datasets in primary breast cancer, treating one as derivation and the other as validation sample. Results are presented for discrimination and calibration. We demonstrate plots of survival probabilities that can assist model evaluation.Our validation methods are applicable to a wide range of prognostic studies and provide researchers with a toolkit for external validation of a published Cox model.

Original publication

DOI

10.1186/1471-2288-13-33

Type

Journal article

Journal

BMC medical research methodology

Publication Date

06/03/2013

Volume

13

Addresses

Hub for Trials Methodology Research, MRC Clinical Trials Unit and University College London, Aviation House 125, Kingsway Country, London WC2B 6NH, UK. pr@ctu.mrc.ac.uk

Keywords

Humans, Breast Neoplasms, Prognosis, Calibration, Models, Statistical, Proportional Hazards Models, Sensitivity and Specificity, Survival Analysis, Reproducibility of Results, Menopause, Time Factors, Adult, Aged, Middle Aged, Female, Kaplan-Meier Estimate