Summary
Like all science, research synthesis is evolving and uncertain. For example, the application of statistical methods for pooling and synthesising the quantitative results of different studies-meta-analysis-is steadily improving though considerable challenges remain (Egger et al., 2002). While much early development was undertaken within the context of systematic reviews of evidence about healthcare interventions-drugs, therapies, technologies-the principles of research synthesis remain the same whatever the subject matter under review.