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Open Access Highly Accessed Research article

External validation of a Cox prognostic model: principles and methods

Patrick Royston1* and Douglas G Altman2

Author Affiliations

1 Hub for Trials Methodology Research, MRC Clinical Trials Unit and University College London, Aviation House 125, Kingsway country London, WC2B 6NH UK

2 Centre for Statistics in Medicine, University of Oxford Wolfson College, Linton Road, Oxford OX2 6UD, UK

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BMC Medical Research Methodology 2013, 13:33  doi:10.1186/1471-2288-13-33

Published: 6 March 2013

Abstract

Background

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.

Methods

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.

Results

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.

Conclusions

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.

Keywords:
Time to event data; Prognostic models; Cox proportional hazards model; External validation; Discrimination; Calibration