Re: Overlapping meta-analyses on the same topic: survey of published studies
The weight and influence of a meta-analysis in determining and qualifying a health care practitioner's deciding method of treatment is tremendous. With much equivocation over the validity of published research , the transparency of clinical guidelines  and the relevance of many risk factors to real world clinical outcomes  - all largely as a result of bias - it would be naive to consider a single conducted meta-analysis as sufficiently qualified, nor should such a practice be endorsed. It is for this reason, that repeated meta-analyses are not only worthy, but they should be actively encouraged as validity stress tests, bringing with them significant assurances to deciding health care practitioners.
In addition to this an overlap of meta-analyses of the same evidence body but differences in methodological approach can be invaluable and shed much insight. This is aptly demonstrated in two meta-analyses, both published in September 2011, that investigated the role of omega 3 fatty acids in major depression [4,5]. Despite the expectedly large duplication of the evidence body examined (12 out of 13 and 12 out of 15 RCTS in the meta analysis published in Molecular Psychiatry and The Journal of Clinical Psychiatry respectively) the conclusions were noticeably different. Through the knowledge gleaned from both these meta analyses we can now be confident in our conclusion that while omega threes as an entity are quite effete in treating major depression, intervention with the sole omega 3 fatty acid EPA, which is antagonised by DHA, can confer significant benefit. Had this overlap of research been discouraged it is likely that many practitioners would still assume a null effect of a very encouraging treatment option.
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2. BMJ 2013;346:f3830
3. BMJ 2013;347:f4247
4. Bloch MH, Hannestad J. Omega-3 fatty acids for the treatment of depression: systematic review and meta-analysis. Molecular psychiatry 2012;17(12):1272-82 doi: 10.1038/mp.2011.100[published Online First: Epub Date Sep 2011].
5. Sublette ME, Ellis SP, Geant AL, et al. Meta-analysis of the effects of eicosapentaenoic acid (EPA) in clinical trials in depression. The Journal of clinical psychiatry 2011;72(12):1577-84 doi: 10.4088/JCP.10m06634[published Online First: Epub Date Sep 2011].
Competing interests: No competing interests