Systematic Reviews: Knowledge Synthesis Tools & Types

Following a systematic review, a meta-analysis of the data from the individual studies included in the SR may be performed.¹ “Meta-analysis is a quantitative, formal, epidemiological study design used to systematically assess the results of previous research to derive conclusions about that body of research.”³ In other words, meta-analysis “statistically pools the results of the individual studies to produce a single estimate of the effect.”⁴ A systematic review and meta-analysis can take between 10-12 months to complete.

Systematic reviews require narrow research questions, while scoping reviews take into account a broad range of research and wider conceptual issues… Scoping reviews gather knowledge on an exploratory research question aimed at mapping key concepts, types of evidence, and gaps in research related to a defined area or filed by systematically searching, selecting, and synthesizing existing knowledge. Frequently scoping reviews are used for “reconnaissance,” to clarify working definitions and conceptual boundaries of a topic or field. Due to the broad nature of scoping review questions, large quantities of literature may need to be reviewed. Scoping reviews often take at least 12 months to complete.

Conducting and adhering to the standards of high quality systematic reviews can be time consuming. The mean time for a systematic review project is around 65 weeks. Rapid reviews may be useful to synthesize current evidence and enable evidence-based decision making in the midst of emergent medical situations, e.g., the Ebola epidemic, avian influenza, and the COVID-19 pandemic. Rapid reviews require transparency and systematic methodologies but curtailing the duration of the review process runs the risk of introducing bias. The rapid review process generally takes 1-6 months.

A review that compiles evidence from multiple reviews into one accessible and usable document. This type of review focuses on a broad condition or problem for which there are competing interventions and highlights reviews that address these interventions and their results. In short, an umbrella review is a systematic review of systematic reviews.

Narrative reviews, or non-systematic literature reviews, are a method of knowledge synthesis, but do not have the stringent methodologies or reporting standards of systematic-type reviews. Narrative reviews do not require a clinically focused research question and inclusion/exclusion criteria are not usually explicitly described. Because a systematic search of the literature is not required for this type of review, a methodology section is not mandatory. Included studies are not evaluated for quality and no risk of bias is performed. Narrative reviews may address more than a single question and are useful for addressing general debates, identifying current lack of knowledge, providing rationales for future research, or speculating on types of interventions available.