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Michigan State University

Nursing Literature Reviews

Step by step guide on how to conduct a literature review.

Levels of Evidence

Levels of evidence (sometimes called hierarchy of evidence) are assigned to studies based on the methodological quality of their design, validity, and applicability to patient care. These decisions gives the grade (or strength) of recommendation. Just because something is lower on the pyramid doesn't mean that the study itself is lower-quality, it just means that the methods used may not be as clinically rigorous as higher levels of the pyramid. In nursing, the system for assigning levels of evidence is often from Melnyk & Fineout-Overholt's 2011 book, Evidence-based Practice in Nursing and Healthcare: A Guide to Best Practice.  The Levels of Evidence below are adapted from Melnyk & Fineout-Overholt's (2011) model. 

Melnyk & Fineout-Overholt (2011)

  • Meta-Analysis: A systematic review that uses quantitative methods to summarize the results. (Level 1)
  • Systematic Review: A comprehensive review that authors have systematically searched for, appraised, and summarized all of the medical literature for a specific topic (Level 1)
  • Randomized Controlled Trials: RCT's include a randomized group of patients in an experimental group and a control group. These groups are followed up for the variables/outcomes of interest. Examples of RCTs are clinical trials that compare the effects of drugs, surgical techniques, medical devices, diagnostic procedures, diets or other medical treatments. (can be Level 2 or Level 4, depending on how expansive the study)
  • Non-Randomized Controlled Trials:  A clinical trial in which the participants are not assigned by chance to different treatment groups. Participants may choose which group they want to be in, or they may be assigned to the groups by the researchers.
  • Cohort Study: Identifies two groups (cohorts) of patients, one which did receive the exposure of interest, and one which did not, and following these cohorts forward for the outcome of interest. (Level 5)
  • Case-Control Study: Involves identifying patients who have the outcome of interest (cases) and control patients without the same outcome, and looking to see if they had the exposure of interest.
  • Background Information/Expert Opinion: Handbooks, encyclopedias, and textbooks often provide a good foundation or introduction and often include generalized information about a condition.  While background information presents a convenient summary, often it takes about three years for this type of literature to be published. (Level 7)