Individual research studies can be useful, but placing too much importance on the conclusions of one study can be misleading. For example, a small study may conclude that red wine prevents breast cancer, but extraordinary health claims like that require evidence from more than one study before it is safe to act on them.
Reviews are valuable sources of evidence because they gather together many individual studies to understand the overall state of the evidence on an issue. Using good quality reviews helps us to avoid being mislead by unreliable studies.
There are two main types of reviews: systematic and narrative.
Systematic reviews and meta-analyses
- Generally considered to be the most valuable form of research evidence
- Collect and evaluate all of the available research on an issue
- Can include meta-analysis of data, where researchers combine data from many studies into one large pool of data with which they can perform statistical analysis
Good systematic reviews are extremely reliable because:
- they use explicit methods that can be easily reproduced by other researchers
- researchers use strict quality criteria to assess the studies
- a published review protocol states exactly how the review will be carried out, so that the review method cannot be changed during the review process
For a more detailed introduction to systematic reviews, see Sense about Science's guide Sense About Systematic Reviews.[external link]
Narrative reviews and journalistic reviews
- Not systematic; do not necessarily include all of the published evidence on an issue
- Give an overview of a topic area
- Researchers read widely on a topic and summarise that evidence
- Do not necessarily include explicit criteria for selecting and appraising evidence
Understanding Health Research