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BMJ No 7129 Volume 316

Letters Saturday 7 February 1998

Bias in meta-analysis detected by a simple, graphical test

Prospectively identified trials could be used for comparison with meta-analyses

Editor,
Egger et al's paper about bias in meta-analysis outlines the value of comparing the results of a meta-analysis of small randomised trials with those of a subsequent large definitive trial.(1) Unfortunately, in many areas of clinical practice such as stroke rehabilitation, large trials are difficult to carry out and unlikely to be available.(2)

One possible solution in this circumstance is to compare the results of meta-analysis with those of prospectively identified trials that could not have been subject to publication bias. This was possible with the recent publication of a systematic review by the Stroke Unit Trialists' Collaboration.(3) The funnel plot for several small trials can be compared with the summary result of either six trials which were identified before they were fully published or two trials (in Perth and Nottingham) which were recruited to the systematic review project before data analysis had started. The figure shows the funnel plot results for individual trials and the summary results for the two groups of prospectively identified trials.

graphicgraphic
Funnel plot results: odds ratio for combined adverse outcomes of death and needing institutional care versus precision of trial or group of trials

RCT=randomised controlled trial

In this case the results of meta-analysis seem to be compatible with those of the prospectively identified trials. With the increasing move towards prospective registration of trials, this approach may allow some assessment of bias in meta-analyses where no large definitive trial is available.

Peter Langhorne Senior lecturer
On behalf of the Stroke Unit Trialists' Collaboration
Academic Section of Geriatric Medicine,
Royal Infirmary,
Glasgow G4 0SF

References

1 Egger M, Davey Smith G, Schneider M, Minder C. Bias in meta-analysis detected by a simple, graphical test. BMJ 1997;315:629-34. (13 September.)

2 Gladman J, Barer D, Langhorne P. Specialist rehabilitation after stroke. BMJ 1996;312:1623-4 .

3 Stroke Unit Trialists' Collaboration. A collaborative systematic review of the randomised trials of organised inpatient (stroke unit) care after stroke. BMJ 1997;314:1151-9.

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