Different levels of evidence


In medicine, Levels of evidence (LoE) are arranged in a ranking system used in evidence based practices to describe the strength of the results measured in a clinical trial or research study (Wikipedia, 2018). A method utilized in evidenced based medicine to determine the clinical value of a study (Moore, n.d.). In grading scheme, scientific evidence can range from level 1 that is the most scientifically valid; to level 5 that is the weakest form of evidence (Sharma, n.d)

Different levels of evidence is categorized as follows:

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Level 1: Randomized controlled trial (RCT) is a study in which the patients are randomly assigned to the treatment or controlled group and are followed prospectively. It can also be in a form of Meta-analysis of randomized trials with homogenous results (Moore, n.d). Sharma (n.d) states that optic neuritis treatment trial study is a perfect example of this level of evidence.

Level 2: Lesser quality RCT is a prospective comparative study. It is a study in which patient groups are separated non-randomly by exposure or treatment, with exposure occurring after the initiation of the study (Moore, n.d). Example of this study according to Sharma (n.d) is a scatter laser photocoagulation for occult choroid neovascularization.

Level 3: Retrospective cohort study pertains to the study in which groups are separated by the current presence or absence of disease and examined for the prior exposure of interest. A perfect example of this level is thrombolytic therapy for acute retinal arterial occlusion (Sharma, n.d).

Level 4: Moore (n.d) states that a level 4 is a Case Series, which means a report of multiple patients with the same treatment, but no control groups or comparison group. Example is macular translocation surgery for the treatment of Chroidal neovascular membrane (CNVM) and Age-related Macular Degeneration (AMD) (Sharma, n.d).

Level 5: Is a case report, or expert opinion, a mere personal observation. An interventional case reports can be classified as level 5 evidence, example is a removal of choroid neovascular membrane (Sharma, n.d).


Meta-analyses –This is a high level of evidence due to the ability of meta-analyses being able to establish statistical significance across studies that may have had contradictory results.  By evaluating multiple studies at one time this increases the statistical significance than with just one study alone. Practice changes in psychology can occur as a result of meta-analyses.  The researcher looks over published studies, and then studies the results for trends.

Experimental studies and Quasi-experimental studies-These types of studies help to determine how effective certain nursing interventions are for patient outcomes.  As a result this could lead to many practice changes in nursing.

Nonexperimental studies-Descriptive and correlational studies are sometimes referred to as nonexperimental and the reason being is that evaluating the study variables is observed naturally and not under any type of control set by the researcher.   

Program Evaluations, RU studies, quality improvement projects, case report.  In our textbook there is an example of a descriptive study design where women were studied with postpartum depression after having pregnancy complications.  The purpose was to describe barriers to treatment, use on internet resources for assistance for postpartum and their inclination for Internet treatment for postpartum depression.

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