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IHW 326: Community Health Promotion: Levels of Evidence

Levels of Evidence

Levels of Evidence (top to bottom): Systematic Reviews, Randomized Controlled Trials, Cohort Studies, Case-Control Studies, Case Series/Case Reports, Editorials/Expert Opinion

Note: the size of each level represents the relative number of publications that might be located in databases, i.e., there are fewer systematic reviews than randomized controlled trials, fewer RCTs than cohort studies, etc.

Image borrowed from Eastern Florida State College Nursing Research Guide


Systematic Reviews focus on a clinical topic and answer a specific question. An extensive literature search is conducted to identify studies with sound methodology. The studies are reviewed, assessed for quality, and the results summarized according to the predetermined criteria of the review question.
Randomized Controlled Trials

carefully planned experiments that introduce a treatment or exposure to study its effect on real patients. They include methodologies that reduce the potential for bias (randomization and blinding) and that allow for comparison between intervention groups and control (no intervention) groups.  A randomized controlled trial is a planned experiment and can provide sound evidence of cause and effect.  

Cohort Studies

identify a group of patients who are already taking a particular treatment or have an exposure, follow them forward over time, and then compare their outcomes with a similar group that has not been affected by the treatment or exposure being studied. Cohort studies are observational and not as reliable as randomized controlled studies, since the two groups may differ in ways other than in the variable under study.

Might also be retrospective, using information that was collected in the past and following the data forward.

Case-Control Studies

studies in which patients who already have a specific condition are compared with people who do not have the condition. The researcher looks back to identify factors or exposures that might be associated with the illness.  They often rely on medical records and patient recall for data collection. These types of studies are often less reliable than randomized controlled trials and cohort studies because showing a statistical relationship does not mean than one factor necessarily caused the other. 

Case Series, Case Reports

collections of reports on the treatment of individual patients or a report on a single patient. Because they are reports of cases and use no control groups to compare outcomes, they have little statistical validity.

Editorials, Expert Opinion consensus conference reports or position statements published by practitioners or experts of acknowledged authority and skill.

Borrowed from the Duke University Medical Center Library & Archives' Introduction to Evidence-Based Practice, and from Marraro, G.A., Yu, R., Liang, Y., & Genovese, U. (2016). Best clinical practice and evidence-based assessment in pediatric ventilation support. Pediatric Clinical Care Medicine, 17(11), 1097-1099.

Types of evidence (Strongest to Weakest)

Meta-Analysis / Systematic reviews

Peer-reviewed quantitative research

Peer-reviewed qualitative research

Text books

Policies and guidelines

Opinion / popular thought

Web information

Qualitative and Quantitative

Qualitative Quantitative
"Quality" of the experience

How do we "quantify" the experience?

The researcher deals with the data in the form of words: narratives of lived experiences

The researcher deals with the data in the form of numbers and statistics.

Subjective: The researcher is the data gathering instrument.

Methods such as focus groups or interviews may be used.

Objective: The researcher uses standardized tools, such as questionnaires or equipment, to collect numerical data.

Subject / Abstract terms: Qualitative Studies; Interviews; Content Analysis; Questionnaires; Focus Groups; Grounded Theory; Ethnography

Subject / Abstract terms: Coefficient; Statistics; Multiple Regression; Two-Tailed Test; ANOVA | 503.943.7111 or 800.841.8261 | 5000 N. Willamette Blvd., Portland, OR 97203-5798
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