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Evidence-Based Practice: Acquire

Evidence Pyramid: Levels of Evidence

The "evidence pyramid" is often used to illustrate the levels of evidence in the literature. When beginning your search for evidence, begin at the highest possible tier. 

Filtered Information is "pre-appraised." This means that the content has been filtered to include studies and reviews that are of higher quality. Keep in mind that the amount of available literature and the number of problems covered gets smaller as you move up the pyramid.

Unfiltered Information represents the original studies. These tiers may not contain studies of high quality and strong evidence, but they cover a much broader range of clinical problems and are much more available.

If you don't know where to begin, consider using a meta-search engine that simultaneously searches multiple websites to find the evidence.

Evidence pyramid showing levels of evidence


Adaption of "Rating System for the  Hierarchy of evidence for Intervention/Treatment Questions" by B. M. Melnyk and E. Fineout-Overholt, 2015, Making the case for evidence-based practice and cultivating a spirit of inquiry, In Evidence-Based Practice in nursing & healthcare: A guide to best practice (3rd ed., pp. 3-23). Wolters Kluwer. Copyright 2015 by Wolters Kluwer Health.

Filtered Information

Filtered information comes in a variety of forms, including systematic reviews, meta-analyses, guidelines (e.g., critically appraised topics).  These works appraise the quality of studies on a particular topic, synthesize the studies' results, and may make a recommendation for clinical practice.

Unfiltered Information

Meta-Search Engines: Searching Multiple Sources

These search engines simultaneously search multiple websites to find the evidence to help you answer your clinical question.

Steps to Acquire Evidence

1. Identify search terms (i.e., keywords and subject headings) based on the answerable clinical question.

2. Identify synonyms or spelling variations for the search terms.

3. Combine key terms using Boolean Operators.

4. Use filters as needed.

5. Modify search as needed to adjust the results of the search.

Glossary of Studies

Definitions of selected types of studies are given below. For additional definitions, consult the references listed below.

Case Report
A detailed report of the diagnosis, treatment, and follow-up of an individual patient. Case reports also contain some demographic information about the patient (for example, age, gender, ethnic origin).

Case Series
A group or series of case reports involving patients who were given similar treatment. Reports of case series usually contain detailed information about the individual patients. This includes demographic information (for example, age, gender, ethnic origin) and information on diagnosis, treatment, response to treatment, and follow-up after treatment.

Case-Control Study
A study that compares two groups of people: those with the disease or condition under study (cases) and a very similar group of people who do not have the disease or condition (controls). Researchers study the medical and lifestyle histories of the people in each group to learn what factors may be associated with the disease or condition. For example, one group may have been exposed to a particular substance that the other was not. Also called retrospective study.

Cohort Study
A clinical research study in which people who presently have a certain condition or receive a particular treatment are followed over time and compared with another group of people who are not affected by the condition.

Randomized Controlled Trial
A controlled clinical trial that randomly (by chance) assigns participants to two or more groups. There are various methods to randomize study participants to their groups.

Agency for Healthcare Research and Quality. Glossary of terms. Retrieved from:
Note: Glossary of terms is no longer available.

National Cancer Institute. Dictionary of cancer terms. Retrieved from:

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