The different types of study used in clinical research can be grouped into what is known as the hierarchy of evidence, with case studies near the bottom and systematic reviews at the top. You were introduced to the hierarchy of evidence (illustrated below) in EBM in Phase 1.
The hierarchy of evidence gives an indication of the quality of evidence, reflecting the reliability and validity of various forms of evidence. There is no universally accepted version of the hierarchy – the figure below illustrates the broad agreement of the order in which the principle types of evidence are typically ranked.
The hierarchy of evidence
The hierarchy of evidence is only one system for ranking the quality of evidence of different study designs. Indeed, numerous different systems to stratify evidence by quality have been developed, primarily for ranking evidence about the effectiveness of treatments/interventions. Different types of research are required for assessing diagnostic accuracy, screening, natural history, prognosis etc., and hence different ‘levels’ of evidence are required. For example, the Oxford Centre for Evidence-Based Medicine suggests levels of evidence (LOE) according to the study designs and critical appraisal of prevention, diagnosis, prognosis, therapy, and harm studies ¹ .
Different systems for ranking or grading the quality of evidence cover criteria such as:
- whether or not the study was adequately randomised
- well-designed vs. poorly-designed
- controlled vs. uncontrolled, etc.
One widely used system for ranking the quality of evidence is known as GRADE – an acronym for Grading of Recommendations Assessment, Development and Evaluation ² . The Scottish Intercollegiate Guidelines Network (SIGN) provides a slightly different approach ³ . The GRADE and SIGN systems both take into account more dimensions than just the quality of medical evidence.
Despite the differences between systems, the purposes are the same: to guide users of clinical research information about which studies are likely to be most valid. It should be noted, though, that the individual studies still require careful critical appraisal.