Chapter 10 – Exploring Heterogeneity – Systematic Reviews in Health Research

Julian P.T. Higgins, Tianjing Li

Abstract

Studies brought together in a meta-analysis are likely to vary in terms of where, when, why, and how they were undertaken. This diversity across the studies often gives rise to heterogeneity: the statistical variability of results beyond what would be expected by chance alone. As a general principle, heterogeneity in meta-analysis should be investigated. In this chapter, we discuss the main two approaches to investigating heterogeneity, which are (i) to group the studies (or participants) into subsets and compare the overall effects across these subsets, an analysis known as subgroup analysis; and (ii) to explore the gradient of effects across studies according to one or more numeric study features, an analysis known as meta-regression. However, subgroup analyses and meta-regression can be problematic for several reasons. Associations between study characteristics and study results are observational and are subject to confounding. Aggregation biases can obscure true associations. Small numbers of studies and numerous potential causes of heterogeneity create a high risk of finding spurious associations. Some analyses that seem appealing, including investigations of small-study effects and baseline risk, can give rise to spurious findings due to statistical artefacts.

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Resources

See practicals below.

Practicals

The instruction file and R script take you through the analysis, presentation of results, and exploration of heterogeneity.

Data, instructions and script Download

Author affiliations

Julian P.T. Higgins

Population Health Sciences, Bristol Medical School, University of Bristol, Bristol, UK
National Institute of Health Research, Applied Research Collaboration West, University Hospital Bristol and Weston NHS Foundation Trust, Bristol, UK

Tianjing Li

Department of Ophtalmology, School of Medicine, University of Colorado Anschutz, Medical Campus, Aurora, CO, USA

How to cite this chapter?

For the printed version of the book

Higgins, J.P.T. and Li, T. (2022). Chapter 10. Exploring heterogeneity. In: Systematic Reviews in Health Research: Meta-analysis in Context (eds M. Egger, J.P.T. Higgins and G. Davey Smith), pp 185-203. Hoboken, NJ : Wiley.