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IJE Advance Access published online on April 17, 2009
International Journal of Epidemiology, doi:10.1093/ije/dyp174
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Published by Oxford University Press on behalf of the International Epidemiological Association © The Author 2009; all rights reserved.

How to assess the external validity of therapeutic trials: a conceptual approach

O M Dekkers1,2,*, E von Elm3,4, A Algra1,5,6, J A Romijn2 and J P Vandenbroucke1

1Department of Clinical Epidemiology, Leiden University Medical Center, Leiden, The Netherlands.
2Department of Endocrinology and Metabolic Diseases, Leiden University Medical Center, Leiden, The Netherlands.
3Institute of Social & Preventive Medicine (ISPM), University of Bern, Bern, Switzerland.
4Department of Medical Biometry and Medical Informatics, University Medical Centre, Freiburg, Germany.
5Julius Center for Health Sciences and Patient Care, University Medical Center Utrecht, Utrecht, The Netherlands.
6Rudolf Magnus Institute, Department of Neurology, University Medical Center Utrecht, Utrecht, The Netherlands.

* Corresponding author. Department of Clinical Epidemiology C7-99, Leiden University Medical Center, PO Box 9600, 2300 RC Leiden, The Netherlands. E-mail: o.m.dekkers{at}lumc.nl


  Abstract

Background External validity of study results is an important issue from a clinical point of view. From a methodological point of view, however, the concept of external validity is more complex than it seems to be at first glance.

Methods Methodological review to address the concept of external validity.

Results External validity refers to the question whether results are generalizable to persons other than the population in the original study. The only formal way to establish the external validity would be to repeat the study for that specific target population. We propose a three-way approach for assessing the external validity for specified target populations. (i) The study population might not be representative for the eligibility criteria that were intended. It should be addressed whether the study population differs from the intended source population with respect to characteristics that influence outcome. (ii) The target population will, by definition, differ from the study population with respect to geographical, temporal and ethnical conditions. Pondering external validity means asking the question whether these differences may influence study results. (iii) It should be assessed whether the study's conclusions can be generalized to target populations that do not meet all the eligibility criteria.

Conclusion Judging the external validity of study results cannot be done by applying given eligibility criteria to a single target population. Rather, it is a complex reflection in which prior knowledge, statistical considerations, biological plausibility and eligibility criteria all have place.

Keywords Clinical trial, external validity

Accepted 3 March 2009


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