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What is evidence?

Understanding what evidence is, and what types of evidence are available, will help make your clinical practice evidence-based.

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What is evidence?

Key Messages

  • best evidence is used alongside clinical judgement and patient preferences
  • best evidence should be of high quality and applicable to your patient and/or patient group

The Oxford Dictionary defines evidence as: 'the available body of facts or information indicating whether a belief or proposition is true or valid'.

Collins Dictionary includes the following: 'data on which to base proof or to establish truth or falsehood'.

Applying this to evidence-based clinical practice, evidence could be described as the available body of facts, information or data on which to base a clinical decision.

What is research evidence?

Research evidence is any fact, information or data provided by a research study. The evidence may be generated from any type of research study utilising any type of research methodology.

Research evidence may come from individual research studies (primary research) or from reviews which combine and analyse the evidence from more than one study on the same topic (secondary research).

Other types of evidence

  • clinical opinion
  • patients, service users and public opinion
  • service audit and evaluation at a local and/or national level
  • datasets

What is best evidence?

This depends on what question you need to answer or decision you are making.

Best evidence should be of high quality and applicable to your patient and/or patient group.

Traditionally, a variety of ‘evidence hierarchies’ have been developed within evidence-based medicine to help clinicians and researcher quickly find the most likely best evidence. They have been criticised, in particular those which judge randomised controlled trials to always be the gold standard.

One of the most useful hierarchies has been developed by the Oxford Centre for Evidence-based medicine (OCEBM). Their most recent version was published in 2011:

access the 2011 version

The OCEBM 2011 levels of evidence is:

  • a hierarchy of the likely best evidence
  • designed so that it can be used as a shortcut for busy clinicians, researchers, or patients to find the likely best evidence

Ideally, every question and/or decision would be answered or made based on a systematic review of all the evidence. In the real world of clinical practice this is unrealistic.

The best type of evidence depends on what you want to know. The OCEBM levels of evidence not only provides a means of assessing the effectiveness of interventions but also to appraise evidence for prevalence, diagnostic test accuracy, prognosis, rare harms, common harms and usefulness of screening.

Further information

OCEBM Levels of Evidence background document describes the history of the development of evidence hierarchies, their criticisms and limitations and future directions:

access the background document

OCEBM Introduction to Levels of Evidence should be read before using the Levels:

access the introduction document

Note a key point at the top of the document: 'no evidence ranking system or decision tool can be used without a healthy dose of judgement and thought'.

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