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Research Guides@Tufts

Community Health

Where to find literature

Levels of evidence

The above "evidence pyramid" shows a hierarchy of the quality of evidence presented in research articles, with the highest quality at the top moving down to the lowest quality at the bottom. 

  • Systematic reviews: involve a comprehensive plan and search strategy to identify, appraise, and synthesis all relevant (and high quality) research on a topic.
    • Meta analyses are a type of systematic review where a statistical analysis has been done to determine the size of the effects.
  • Clinical guidelines: based on the evidence presented by many studies, gives best practices
  • Randomized control trials: participants are randomly placed in treatment or control groups for experimental comparison. This is the best type of study for evaluating the effects of interventions.
  • Cohort studies: longitudinal data from groups who have non-randomly been placed into treatment or control groups. This is the best type of study for evaluating the link between risk factors and outcomes.
  • Case studies: examination of individuals or groups with a certain illness/problem/outcome of interest (usually compared to a matched group of controls). May be qualitative or quantitative. Used to study the causes of an illness.

For more information which type of evidence is best for your needs, see the Center for Evidence-Based Medicine website: http://www.cebm.net/ocebm-levels-of-evidence/