The aim of secondary research is to produce a more or less systematic appraisal and/or synthesis of the existing primary research on a topic. There are numerous types of reviews which aim to summarise or synthesise the evidence on a topic, but here we will focus on two: meta-analyses and systematic reviews.
For a fuller discussion of the range of review types, their features and uses, see: Sutton, A. et al. (2019) 'Meeting the review family: exploring review types and associated information retrieval requirements', Health Information and Libraries Journal, 36 (3), pp. 202-222. doi:10.1111/hir.12276
A meta-analysis is a statistical synthesis of the results from multiple individual studies, usually randomised controlled trials (RCTs),
Carrying out a meta-analysis of studies allows results from multiple studies looking at the effect of an intervention to be combined, allowing for greater precision in the estimation of effects, and clarity over the direction and size of an effect. A meta-analysis can provide more conclusive evidence for or against the effectiveness of an intervention than individual studies alone.
A good meta-analysis should always be based on a systematic review of studies, and requires some homogeneity of participants, settings, interventions and outcome measures in the studies included.
A systematic review is not simply a literature review. A systematic review is a study which aims to synthesise all of the available primary research on a specific topic. The first step in a systematic review is a thorough search of all appropriate sources, including subject related databases, clinical trial registers and grey literature, in order to identify all of the relevant evidence. These searches should ideally be carried out by a librarian or information specialist in the field, or by others with a similar level of expertise. The systematic review itself should be carried out by two or more researchers, as a means of reducing possible bias.
All identified studies are screened for inclusion or exclusion according to strict criteria set out at the start of the study, and the data from those studies selected for inclusion is analysed and synthesised. Part of this process is an attempt to identify any potential source of bias in existing findings. A systematic review will offer a summary of the available research findings, and offer conclusions on the basis of these, taking into account any flaws or limitations in the original studies.
A systematic review can offer more generalisability and consistency of research findings than the individual studies on which it is based.
Systematic reviews may employ quantitative, qualitative (experiential), or mixed-methods approaches.