Level 1: Systematic review & meta-analysis of randomized controlled trials; clinical guidelines based on systematic reviews or meta-analyses
Level 2: One or more randomized controlled trials
Level 3: Controlled trial (no randomization)
Level 4: Case-control or cohort study
Level 5: Systematic review of descriptive & qualitative studies
Level 6: Single descriptive or qualitative study
Level 7: Expert opinion
Modified from: 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.
When you start researching you will encounter many different types of evidence such as systematic reviews, randomized controlled trials, clinical guidelines and opinion articles. These different resources will not all have the same "weight" in terms of reliability and trustworthiness.
Evidence Level hierarchies are systems used to rank evidence according to certain criteria. There are many hierarchies, including the examples on this page.
Source: Hopp, L., & Rittenmeyer, L. (2012). Introduction to evidence-based practice: A practical guide for nursing. Philadelphia: F.A. Davis.
Evidence Levels, or Hierarchies, are systems used to rank evidence according to certain criteria.
What's Not in the Included in the Levels of Evidence?
There are some types of informational sources that are not included in the levels of evidence model you are using for your class. For example, integrated reviews and literature reviews are not considered to be rigorously evidence-based sources according to the JBI Levels of Evidence. Therefore, if you are using JBI's LOE model for a research assignment, you will need to locate articles that can be categorized as evidence-based according to JBI.
While informational sources may be useful for your understanding, you should not cite them as scientific evidence unless the LOE model you are using includes them in their levels of evidence. Examples of informational sources may include: Informative articles, trade journal, magazine, and news articles, blogs, and popular sites such as Wikipedia.
"A controlled trial means that study participants are split into two groups: One group is given the treatment and the other (the control group) is not. The control group may be given a placebo that mimics the actual treatment, but does not contain the treatment being tested.
For example, a sugar pill or an injection of saline solution may be used instead of a dose of the drug. This ensures the only meaningful difference between the two groups is whether they received the treatment or not. The control group helps researchers learn what would have happened to the treatment group if they hadn’t received the treatment. For example, some patients may recover on their own. Researchers need to know how often this happens, so they don’t attribute all recoveries to the effect of the treatment.
Study participants are randomly assigned to one group or the other, a process similar to a coin toss. Just as a coin toss is equally likely to end up heads or tails, study participants are equally likely to end up in the treatment or the control group. With enough study participants, this results in two groups that closely resemble each other. The only difference is that one group got “heads” while the other got “tails.”
The randomization of randomized-controlled trials with large enough samples ensures that all possible differences are accounted for, even those that may not be observed, such as genetic traits. If the treatment and control groups are similar at the start of the study but end up with different outcomes, the treatment is the most likely cause. The randomized-controlled trial allows researchers to rule out alternative explanations."
From "What is a clinical trial? A health policy expert explains" in Credo Reference.Content adapted with permission from Grand Canyon University Library: https://libguides.gcu.edu/EvaluatingSources
Systematic Reviews or Meta-Analysis
Randomized Controlled Trials
Clinical Practice Guidelines
Nonrandomized Controlled Trials
Cohort Study or Case-Control Studies
Meta-Synthesis of Qualitative or Descriptive Studies
Qualitative or Descriptive Studies
Expert Opinion
Synthesis of Cohort Study or Case-Control Studies
Single Cohort Study or Case-Control Studies
Meta-Synthesis of Qualitative or Descriptive Studies
Single Qualitative or Descriptive Studies
Expert Opinion
Meta-synthesis of Qualitative Studies
Cohort Studies
Single Qualitative studies
Synthesis of Descriptive Studies
Single Descriptive Studies
Expert Opinion
Randomized Controlled Trials
Cohort Studies
Case Control
Case Series
Economic Analysis