Skip to Main Content

University Library

Answering Clinical Questions: Hierarchy of Evidence

Clinical evidence based practice resources

Banner image sources left to right:
Louis Pasteur Public Domain; Skeleton Foot Image by Mivervaa (GNU Free Documentation License, version 1.2); Barber-Dentist, 16th century, Portugal Public Domain; Hippocrate Public Domain; Bore Track of Strzelecki Desert, South Australia Photograph by Kdliss (GNU Free Documentation License, version 1.2)

What is the Hierarchy of Evidence?

Different types of health and medical information are often represented within a hierarchy of evidence, with ‘single studies’ placed at the base of the pyramid and levels of analysis and synthesis of these primary research outputs represented as higher levels of evidence. 

Some defintions of different levels of evidence are provided to the right -->

There are different interpretations of these hierarchies of synthesised medical evidence, and of where different types of primary studies sit on this hiearchy, as outlined below. 

Evidence-based guidelines and summaries aim to provide a broader holistic overview of inter-related health issues and factors related to clinical topics or that may impact the diagnosis and treatment of specific medical conditions.  They are organised in ways that aim to make them useful as decisioin making tools within clinical practice.

 

Different Hierarchies of Evidence

In first pyramid of evidence diagram, placement within the hierarchy is based on the level to which the focus is on answering a single medical research question – such as a specific treatment intervention - versus a broader exploration of multiple health questions and factors related to a medical condition.

In the second pyramid of evidence diagram, systematic reviews have been placed as the highest form of evidence; in this instance. They likely do reflect the highest form of evidence on evaluating all primary studies related to a highly specific health research question or medical intervention.

 

Hierarchy of Single Studies

The 'single case studies' idenfified at the lower levels of the syntheised hierarchy of evidence pyramids are also often represented as a hierarchy. Randomised Control Trial (RCT) studies are often represented as the highest level of evidence given the rigourous methodologies used to minimise interpretive bias of data collected.

However, decisions on the highest or most appropriate level of evidence will be impacted by your specific health issue or medical condition inquiry. For instance, for some health related research questions a RCT study may not be available or appropriate. 

Pyramid of single studies research types.

Definitions of Levels of Evidence

Decision Support Systems provide management guidance by linking individual patient information to the best available evidence from relevant research.

Evidence Based Summaries draw on systematic reviews or the best available primary studies to integrate the highest level evidence available.

Evidence Based Guidelines use evidence drawn from systematic reviews and studies. They are problem focused and may take into account resources and practice relevant to the organisation sponsoring the guideline development.

Systematic Reviews use a transparent process to identify, appraise and summarise studies in relation to a defined clinical question.  A systematic review may or may not include a meta-analysis - a summary of the medical literature that used statistical techniques to combine study results.  Ideally a meta-analysis should be done in the context of a systematic review.

Synopses of Single Studies critically appraise individual articles (primary studies) and include a summary of key methods and findings of the paper and a critique or commentary related to clinical practice.

Single Studies are primary research including:

  • Randomised Control Trials (RCTs)
  • Cohort studies
  • Case-controlled studies
  • Case series/case reports
  • Qualitative studies

CONTENT LICENCE

 Except for logos, Canva designs, AI generated images or where otherwise indicated, content in this guide is licensed under a Creative Commons Attribution-ShareAlike 4.0 International Licence.