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Nursing: A Guide to Library Research

Levels of Evidence

Evidence based pyramid

Level 1 - Articles describing systematic reviews or meta-analyses of randomized controlled trials; clinical guidelines based on systematic reviews or meta-analyses

Level 2 - Articles reporting well-designed randomized controlled trials

Level 3 - Articles reporting controlled trial, no randomization

Level 4 - Articles reporting on case-control or cohort study

Level 5 - Articles reporting systematic reviews of descriptive & qualitative studies

Level 6 - Articles reporting on single descriptive or qualitative study

Level 7 - Articles reporting expert opinion


Melnyk, B.M. & Fineout-Overholt, E. (2015). "Box 1.3: Rating system for the hierarchy of evidence for intervention/treatment questions" in Evidence-based practice in nursing & healthcare: A guide to best practice (3rd ed.) (pp. 11). Philadelphia, PA: Wolters Kluwer Health.

Search Strategies for Finding Evidence

Subject search results can be limited by study types to locate research based on level of evidence.  Below are tips for utilizing "limits"or "filters" for the following databases:

CINAHL Complete

To limit your subject search results by research study types, in the Search Options section in the Basic or Advanced search, find the “Publication Type” limit,  and select the desired type of study, such as Clinical TrialRandomized Controlled Trial or Systematic Review.  Hold down the “Ctrl” key to select more than one publication type.  Once you have selected the desired publication types, click Search.

For study types that are not listed in the “Publication Type” limit box, search the “CINAHL Headings” for the study type.  For example, search for Prospective Studies (also called cohort studies) as a CINAHL heading, and use the AND operator to combine this set of results with your topic search result.


To limit your subject search results by research study types, at the top of the left margin of your search results, under Article Types, click the type of study you want to find. If the desired type of study is not displayed, click "Customize" and choose from the resulting list all the study types you would like to display as available filters, then click "Show", and the selected study types will display in the left margin.

Click each type of study you would like included in your in your results, holding down the Control key to select more than one type.  Note: "Clinical Trial" will include all individual phases of clinical trials, and will also include Randomized Controlled Trials.

For study types that are not listed as Article Types, like "cohort studies" or "longitudinal studies," search these terms as MeSH terms, and use the AND operator to combine with your topic search results.