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Research Methods & Reporting

Rethinking pragmatic randomised controlled trials: introducing the “cohort multiple randomised controlled trial” design

BMJ 2010; 340 doi: https://doi.org/10.1136/bmj.c1066 (Published 19 March 2010) Cite this as: BMJ 2010;340:c1066

Rapid Response:

The cmRCT

Relton et al 1 propose an interesting study design for pragmatic
trials which may overcome some of the drawbacks of other RCTs. However,
the cmRCT does seem to have several limitations:

• Like most pragmatic trials, it does not control for non-specific
effects of the experimental treatment 2.

• It is open to selection bias: only patients in the randomized
arm(s) need to consent to the experimental treatment. They are thus
selected and possibly different from the pool of all patients.

• The comparator treatment is ‘care as usual’; this means trialists
have little control over what exactly patients elect for (self-)treatment.
Thus the experimental treatment might be tested against a package of care,
the exact contents of which is unknown. Essentially one might compare one
unknown to another.

These drawbacks might mean that the cmRCT generates false positive
results. I can, for instance, imagine a pure placebo, like homeopathy,
coming out of such a test smelling of roses.

E. Ernst

Reference List

1) Relton C, Togerson D, O'Cathain A, Nicholl J. Rethinking pragmatic
randomised controlled trials: introducing the "cohort multiple randomised
controlled trial" design. BMJ 2010; 340:963-967.

2) Ernst E, Lee MS. A trial design that generates only ''positive''
results. J Postgrad Med 2008; 54(3):214-216.

Competing interests:
None declared

Competing interests: No competing interests

10 May 2010
Edzard Ernst
Director, Complementary Medicine
Peninsula Medical School, 25 Victoria Park Road, EX2 4NT