Evidence explained

What is evidence

Evidence is the results of research used to support decision making and can be broadly categorised into two main categories in evidence based practice:

  • Filtered (secondary) information - provides analysis, synthesis, interpretation, commentary and/or evaluation of original research studies (unfiltered information) and often make recommendations for practice. These are considered the highest quality of evidence
  • Unfiltered (primary) information - contains original data and analysis from research studies with no external appraisal or interpretation provided

The Evidence Pyramid

Medical researchers and practitioners rank this evidence according to its quality. These levels when pictured together form a pyramid with the highest quality evidence at the top of the pyramid as there is less of this available. This pyramid is often referred to as the "Evidence Pyramid".

EBM Pyramid © 2006 Trustees of Dartmouth College and Yale University. All rights reserved.

Filtered (secondary) information

The top three levels include filtered sources of information. These include (from highest to lowest quality evidence):

  • Systematic reviews - the application of strategies that limit bias in the assembly, critical appraisal, and synthesis of all relevant studies on a specific topic. meta-analysis may be, but is not necessarily, used as part of this process. Systematic reviews focus on peer-reviewed publications about a specific health problem and use rigorous, standardised methods for selecting and assessing articles. A systematic review differs from a meta-analysis in not including a quantitative summary of the results.
    (From Dictionary of Epidemiology 5th edition - definition of review, systematic)
  • Critically-appraised topics (CATs) and articles - a short summary of evidence or summary of an individual article from the literature, created to answer a specific clinical question.
    (From Oxford University Centre for Evidence based Medicine Glossary)

Unfiltered (primary) information

The next three lower levels of the pyramid consist of unfiltered sources of information. These include (from highest to lowest quality evidence):

  • Randomised control trials - an epidemiological experiment in which subjects in a population are randomly allocated into groups, usually called study and control groups, to receive or not to receive an experimental preventive or therapeutic procedure, maneuver, or intervention. The results are assessed by rigorous comparison of rates of disease, death, recovery, or other appropriate outcome in the study and control groups.
    (From Dictionary of Epidemiology 5th edition - definition of randomised control trial)
  • Cohort studies - the analytic epidemiological study in which subsets of a defined population can be identified who are, have been, or in the future may be exposed or not exposed, or exposed in different degrees, to a factor or factors hypothesised to influence the occurrence of a given disease or other outcome. The main feature of cohort study is observation of large numbers over a long period (commonly years), with comparison of incidence rates in groups that differ in exposure levels.
    (From Dictionary of Epidemiology 5th edition - definition of cohort study)
  • Case-controlled studies and case series
    • a case-control study is the observational epidemiological study of persons with the disease (or another outcome variable) of interest and a suit-able control group of persons without the disease (comparison group, reference group). The potential relationship of a suspected risk factor or an attribute to the disease is examined by comparing the diseased and nondiseased subjects with regard to how frequently the factor or attribute is present (or, if quantitative, the levels of the attribute) in each of the groups (diseased and nondiseased).
      (From Dictionary of Epidemiology 5th edition - definition of case-control study)
    • Case series are a collection of patients with common characteristics used to describe some clinical, pathophysiological, or operational aspect of a disease, treatment, or diagnostic procedure.
      (From Dictionary of Epidemiology 5th edition - definition of case series)

Background information

The foundation level of the pyramid consists of background information and expert opinion. This is the lowest level of scientific quality and is not considered evidence as such.