The purpose of meta-analysis

by Harri Hemilä

This text is based on p 28 of Hemilä (2006)
These documents have up to date links to documents that are available via the net.
Harri Hemilä
Department of Public Health
University of Helsinki, Helsinki, Finland
harri.hemila@helsinki.fi

Home: http://www.mv.helsinki.fi/home/hemila

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Version May 29, 2012


In general, ‘meta-analysis’ denotes systematic and thorough investigation of scientific literature on a specific topic, and combining the results of ‘close enough’ studies by statistical formulae, but there is substantial difference of opinion as to how people see the coverage of the term ‘meta-analysis.’ The term was coined by Gene Glass to describe the process of synthesizing results from separate but similar studies (Mann 1990).

Interestingly, the origin of the meta-analytic approach was connected to vitamin C (Hampton 2002; Milne & Chalmers 2004), since in his treatise on vitamin C deficiency, James Lind carried out a systematic search of all the older literature and wrote that "As it is no easy matter to root out old prejudices, or to overturn opinions which have acquired an establishment by time, custom and great authorities; it became therefore requisite for this purpose, to exhibit a full and impartial view of what has hitherto been published on the scurvy; and that in a chronological order, by which the sources of those mistakes may be detected. Indeed, before this subject could be set in a clear and proper light, it was necessary to remove a great deal of rubbish" (1753 p 7; see Appendix 1).

Typically, meta-analysis is used (1) to increase statistical power for primary end points and for subgroups, or (2) to improve estimates of the size of the effect, or/and to (3) resolve uncertainty when reports disagree (Sacks et al. 1987). The purpose of increasing statistical power emerges from the problem that a large proportion of controlled trials are so small that they cannot provide meaningful evidence for the effectiveness or otherwise of therapy, simply because the confidence intervals (CI) are very wide. This problem of low statistical power was illustrated by Freiman, Chalmers, et al. (1978) who analyzed 71 ‘negative’ trials (P > 0.05 for the difference of interest), showing that 50 of them could have missed a 50% benefit because the trials were simply too small. Thus, meta-analysis can be used to enhance the use of data from small studies with ambiguous results by combining the results of several to test whether there is any overall evidence of effect, and to estimate its magnitude. This is the most common use of meta-analysis.

When the optimism on the fruitful opportunities provided by meta-analysis was high, Chalmers’ group believed that "A quantitative synthesis of the data in similar randomized controlled trials can potentially be more useful to the practicing physician than a traditional narrative review article, but such a synthesis must be properly performed to warrant serious attention" (Sacks et al. 1987). Thomas Chalmers also commented that "Meta-analysis is the wave of the future. The days of the expert supposedly putting the state of the field into a review article are numbered" (Mann 1990).

References

Freiman JA, Chalmers TC, Smith H, Kuebler RR (1978) The importance of beta, the type II error and sample size in the design and interpretation of the randomized control trial: survey of 71 “negative” trials. N Engl J Med 299:690-4  PubMed

Hampton JR (2002) Evidence-based medicine, opinion-based medicine, and real-world medicine. Persp Biol Med 45:549-68

Lind J (1753) A Treatise of the Scurvy, in Three Parts, Containing an Inquiry into the Nature, Causes, and Cure of That Disease. Together with a Critical and Chronological View of What Has Been Published on the Subject. Edinburgh: Sands, Murray and Cochran. 456 pp. Republished in: (1953) Lind’s Treatise on Scurvy [Stewart CP, Gutrie D, eds]. Edinburgh, UK: Edinburgh University Press [pages in the current thesis refer to the 1953 reprint]

Mann C (1990) Meta-analysis in the breech. Science 249:476-80 * see also:  garfield.library

Milne I, Chalmers I (2004) Documenting the evidence: the case of scurvy. Bull WHO 82:791-2  * see also: (2004);82:793-6 

Sacks HS, Berrier J, Reitman D, Ancona-Berk VA, Chalmers TC (1987) Meta-analyses of randomized controlled trials. N Engl J Med 316:450-5
  PubMed

 


Copyright: © 2006-2009 Harri Hemilä. This text is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.  

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Vitamin C and infections in animals by Harri Hemilä is licensed under a Creative Commons Attribution 1.0 Finland License.
Based on a work at www.mv.helsinki.fi/home/hemila/metaanalysis



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