Education and debate

Meta-analysis: Unresolved issues and future developments

^a Department of Social Medicine, University of Bristol, Bristol BS8 2PR

Correspondence to: Professor Davey Smith zetkin@bristol.ac.uk

	Introduction

Since its recent introduction into clinical epidemiology, meta-analysis has established itself as an influential branch of biostatistics. Several books have focused mainly or entirely on meta-analysis in medicine,^{1 2 3 4 5} and the latest editions of relevant textbooks generally include a section on meta-analysis.^{6 7 8 9} Computer software entirely devoted to meta-analysis has been developed, and meta-analytic procedures have been introduced in general statistical software packages. We will soon be providing an overview of software packages on the BMJ's website.¹⁰ Several unresolved issues concerning meta-analysis remain, and in this final article of our series we address some of the topics that are likely to feature in future discussions of the appropriate practice and domain of meta-analysis.

	Should unpublished data be included in meta-analyses?

Publication bias, discussed in detail in a previous article,¹¹ is a major threat to the validity of meta-analysis. Obtaining and including data from unpublished . . . [Full text of this article]

Summary points

Box 1: Controversy over selective serotonin uptake inhibitors and depression

	Subjectivity in data analysis and reporting

	Individual patient data or summary statistics—which should be included in a meta-analysis?

Box 2: Coronary artery bypass graft surgery and survival: meta-analysis using individual patient data

	Value of "failed" meta-analyses

	The Cochrane Collaboration

	Clinical application of results from meta-analyses

	Outlook

	Acknowledgements

	References

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