Not all evidence is the same and therefore cannot be treated as equal. Systematic reviews always consider the quality of evidence in each research study before summarising the information in a review. Evidence based medicine requires the use of the best available evidence, in order to be able to make effective recommendations.
(Greenhalgh, 2019, p.42-43) lists the traditional hierarchy of evidence in the following order:
1. Systematic reviews and meta‐analyses
2. RCTs with definitive results
3. RCTs with non‐definitive results
4. Cohort studies
5. Case-control studies
6. Cross‐sectional surveys
Cardiff University Library Services (no date) have also created an evidence pyramid, which illustrates the hierarchy of evidence based on quality. At the top of the pyramid is the most reliable and current evidence and at the bottom is the evidence which is regarded as the lowest quality, which includes background information or expert opinion.
