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Evidence Based Practice: 5 A's

Evidence Pyramid

The Evidence Hierarchy: What is best evidence?

Evidence hierarchies rank studies based upon the rigor (validity and reliability) of their research methods. The more rigorous the methodology the less risk of bias, which leads to the highest quality of evidence. 

the structure of the evidence based pyramid is described in the text below this image

Study designs at the top of the evidence pyramid are considered to have more rigorous methodologies and are more likely to minimize the effect of bias on the results of the study. 

"Evidence Pyramid" by Dartmouth Biomedical Libraries is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Synthesized evidence

Synthesized evidence, also known as filtered, presents evidence from multiple studies, which has been critically appraised by experts for its rigor (validity and reliability). This type of evidence is appropriate for clinical point-of-care decision-making. Sources of synthesized evidence can be found within systematic reviews, meta analyses and evidence-based clinical practice guidelines.

Benefits of synthesized research: 

  • Multiple studies (primary research) on a topic have been summarized
  • Studies within the synthesized research have been critically analyzed
  • Less risk of bias
  • Larger data pool

Limitations of synthesized research: 

  • There may not be a systematic review on the topic of inquiry
  • The process of synthesized research can take years to complete and therefore may not be current
  • A well conducted Randomized Control Trial (RCT) may provide higher quality evidence than a systematic review of lesser quality studies.

Examples of synthesized evidence: 

  • Systematic Review: "A systematic review is a type of research that looks at the results from all of the good-quality studies. It puts together the results of these individual studies into one summary. This gives an estimate of a treatment's risks and benefits. Sometimes these reviews include a statistical analysis, called a meta-analysis, which combines the results of several studies to give a treatment effect."
  • Meta Analyses: "A meta-analysis provides a weighted average of all the results from each of the RCTs included. It yields an overall statistic (together with its confidence interval) that summarizes the effects of the experimental intervention compared with a control intervention on a specific clinical outcome."

What is the best evidence and how to find it. BMJ Best Practice. (n.d.). Retrieved September 16, 2021, from

Haidich A. B. (2010). Meta-analysis in medical research. Hippokratia14(Suppl 1), 29–37.

Primary evidence

Primary evidence, also know as unfiltered, are original single studies. This type of evidence has not been synthesized or aggregated with similar study results. Examples include Randomized Control Trials (RCTs), Cohort studies, Clinical trials, and Case-control.

Benefits of primary evidence: 

  • More timely, may provide more recent evidence
  • More likely to locate research on a specific topic
  • Accepted as strong evidence if synthesized evidence is not available
  • A large, well conducted Randomized Control Trial (RCT) may provide stronger evidence than synthesized evidence of weaker trails. 

Limitations of primary evidence: 

  • A lower level of strength due to the higher risk of bias and possible presence of confounding factors. 
  • It has not been summarized and may require more in depth reading
  • It has not been compared to similar primary studies of the same research 
  • Requires critical appraisal prior to implementation

Examples of Primary evidence: 

  • Randomized Control Trial: an experimental comparison study which distributes participants to intervention or placebo groups through randomization methods, usually double or triple blinded. RCTs are considered the best type of study to determine the effects of an intervention. 
  • Cohort studies: (Prospective) An exposure is defined. Participants are compared with a control group to determine if an outcome is developed.
  • Clinical trials: Aimed at evaluating a medical, surgical, or behavioral intervention. This type of trial is used to determine if a new treatment is more effective than a standard treatment and to determine possible side effects. 
  • Case control studies: (Retrospective) A group with a predefined outcome is compared to a group free of the outcome. The investigation is retrospective because it looks back in time to determine degree and time of exposure. 

Question Domain/Type of Evidence

Question Domain Description Evidence Level
Therapeutic/Treatment Effectiveness of intervention: treatment, medication, surgical procedure, exercise, lifestyle change Randomized Control Trial (RCT)
Prevention Effectiveness of an intervention or exposure to prevent mortality. Assessment of preventative measures and evaluation of potential harms/benefits RCT or Prospective Study
Diagnosis Measures the ability of a test or procedure to differentiate between diagnostic capabilities RCT or Cohort Study
Prognosis Measures the ability to forecast a probable cause of disease or illness development Cohort Study or Case-Control Series
Etiology  Measures the harmful effects or an intervention or exposure Cohort Study
Meaning Documents patients' experiences or concerns Qualitative Study


Synthesized Evidence Sources

Synthesized resources: Meta-Search Engines: 

Meta search engines simultaneously search multiple evidence-based resources

Synthesized evidence: Systematic Reviews and Meta-Analyses:

Synthesized evidence: Critically-appraised topics (Evidence synthesis & Guidelines)

Authors of critically-appraised topics evaluate and synthesize multiple research studies

Primary evidence sources

To access specific types of studies within these databases, use the built-in search filters.