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Rapid response to:

Research Methods & Reporting

The Cochrane Collaboration’s tool for assessing risk of bias in randomised trials

BMJ 2011; 343 doi: (Published 18 October 2011) Cite this as: BMJ 2011;343:d5928

Rapid Response:

External validity: On the back burner - again

To the Editor: Randomised controlled trials (RCTs) are considered to
be the gold standard of clinical research and the building blocks of most
systematic reviews and practice guidelines. Yet, the recently published
recommended criteria for assessing risk of bias in RCTs by the Cochrane
Collaboration are almost exclusively concerned with internal
validity.<1> Although the recommended standards are both timely and
welcomed, the perennial problem with poor uptake of clinical practice
guidelines, especially in primary care settings, is an on-going neglect of
the external validity or applicability in the design and conduct of
randomized controlled trials (RCTs). Some of the frequently identified
threats to external validity include narrow inclusion criteria, use of run
-in periods, and length of follow-up or clinical relevance of the primary
end-points used. <2,3,4>

Although, as stated by the authors, applicability is less relevant
without internal validity, the opposite is equally true. It is
disappointing to see that the external validity, once gain, was placed on
the back burner. Internal and external validity are equally important, and
both ought to be maximized in order to generate the evidence that is both
valid and applicable. This is not a zero-sum game.


1. Higgins JPT, Altman DG, Peter C G?tzsche PC, et al. The Cochrane
Collaboration's tool for assessing risk of bias in randomised trials. BMJ
343:d5928. 2011

2. Fortin M, Dionne J, Pinho G, Gignac J, Almirall J, Lapointe L.
Randomized controlled trials: do they have external validity for patients
with multiple comorbidities? Ann Fam Med 4 104-8. 2006.

3. Rothwell PM. External validity of randomised controlled trials:
"To whom do the results of this trial apply?" Lancet 365 82-93. 2005.

4. Glasgow RE, Lichtenstein E, Marcus AC. Why don't we see more
translation of health promotion research to practice? Rethinking the
efficacy-to-effectiveness transition. Am J Public Health 93, 1261-1267.

Competing interests: No competing interests

31 October 2011
Janusz Kaczorowski
Professor & Research Director
University of Montreal