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Levels of Evidence
Evidence-based practice is a conscientious, problem-solving approach to clinical
practice that incorporates the best evidence from well-designed studies, patient
values and preferences, and a clinician’s expertise in making decisions about a
patient’s care. Unfortunately, no standard formula exists for how much these factors
should be weighed in the clinical decision making process. However, there are a
variety of rating systems and hierarchies of evidence that grade the strength or
quality of evidence generated from a research study or report. Being knowledgeable
about evidence-based practice, and levels of evidence, is important to every clinician
as clinicians need to be confident about how much emphasis they should place on
a study, report, practice alert or clinical practice guideline when making decisions
about a patient’s care.
Gannett Education’s Rating System:
The levels of evidence listed here have been developed with the help of nurse
experts and other industry resources. We thank those who have contributed to making
our system relevant and applicable to determining the levels of evidence that support
our CE publications.
Evidence-based information ranges from Level A (the strongest) to Level C (the weakest).
In 2013, we have added Level ML, multilevel, to identify clinical practice guidelines
that contain recommendations based on more than one level of evidence:
control trials: the classic “gold standard” study design. In RCTs, subjects
are randomly selected and randomly assigned to groups to undergo rigorously controlled
experimental conditions or interventions.
Systematic review or meta-analysis
of all relevant RCTs. A systematic review is a critical assessment of existing evidence
that addresses a focused clinical question, includes a comprehensive literature
search, appraises the quality of studies and reports results in a systematic manner.
Meta-analysis a study design that uses statistical techniques to combine and analyze
data from many RCTs.
practice guidelines: based on systematic reviews of RCTs. Evidence-based clinical
practice guidelines provide the strongest level of evidence to guide clinical practice
because they are based on rigorous reviews of the best evidence on specific topics.
control trials without randomization: In this type of study, random assignment
is not used to assign subjects to experimental and control groups. Therefore, this
type of research is less strong in internal validity because it can’t be assumed
the subjects in the study are equal on major demographic and clinical variables
at the beginning of the trial. Frequent problems with this type of study include
intentional or unintentional bias in sample enrollment; nonblinding, unclear criteria
for participant selection; or unreliable or invalid tools.
cohort study: an examination of groups of people who have common characteristics
or exposure experiences to compare outcomes in those exposed vs. outcomes in those
not exposed (e.g., development of heart disease after exposure or nonexposure to
10 years of secondhand smoke).
study: use of an observational approach in which subjects known to have a disease
or outcome are compared with subjects known not to have that disease or outcome.
Subjects are matched on characteristics so that they are as similar as possible
except for the disease or outcome. Case-control studies are generally designed to
estimate the odds (using an odds ratio) of developing the studied condition or disease
and can determine if an associated relationship exists between the condition/disease
and risk factors.
study: studies that do not control participant selection or interventions (e.g.,
a convenience sample, such as patients on a given unit, may be studied because it’s
the only group reasonably available).
study: studies that observe people over a long time to determine risk or likelihood
of developing diseases. These studies include retrospective database searches or
prospective studies that follow a population over time.
study/quantitative study: descriptive, word-based phenomena, such as symptoms,
behaviors, culture and group dynamics. Quantitative studies use statistical methods
to establish numerical relationships that are correlational or cause and effect.
Consensus viewpoint and expert opinion: a study that obtains agreement about
specific practices from all clinical experts on a review panel. Expert opinion involves
obtaining agreement from a majority of clinical experts on a review panel. Note:
This level of evidence is used when there are no quantitative or qualitative studies
in a particular area.
Meta-synthesis: a systematic review that synthesizes findings from qualitative
studies using an interpretive technique to bring small study findings, such as case
studies, to clinical application.
More than one level of evidence as defined in Gannett Education’s rating system.
Evidence-based Practice Resources:
Agency for Healthcare Research and Quality Evidence-based Practice Centers(ahrq.gov/clinic/epc )
The Cochrane Collaboration:
o Cochrane Reviews (cochrane.org/cochrane-reviews)
o Evidence-based healthcare (cochrane.org/about-us/evidence-based-health-care)
Alfaro-LeFevre R. Critical Thinking, Clinical Reasoning, and Clinical Judgment: A
Practical Approach. 5th ed. St. Louis, MO: Elsevier-Saunders; 2013.
Ebell MH, Siwek J, Weiss BD, et al. Strength of recommendation taxonomy (SORT):
a patient centered approach to grading evidence in the medical literature. Am Fam
Physician. 2004;69(3):548-556. http://www.aafp.org/afp/2004/0201/p548.html.
Published February 1, 2004. Accessed January 9, 2013.
Evidence-based medicine toolkit. American Academy of Family Physician Web site. http://www.aafp.org/online/en/home/publications/journals/afp/ebmtoolkit.html.
Accessed January 9, 2013.
Is all evidence created equal? University of Illinois at Chicago University Library
Web site. http://www.uic.edu/depts/lib/lhsp/resources/levels.shtml Updated March
7, 2008. Accessed January 9, 2013.
Levels of evidence. Centre for Evidence-Based Medicine Web site. http://www.cebm.net/index.aspx?o=1025.
Updated April 15, 2011. Accessed January 9, 2013.
Melnyk BM, Fineout-Overholt E. Evidence-Based Practice in Nursing & Healthcare. A
Guide to Best Practice. Philadelphia, PA: Lippincott Williams & Wilkins.
Newhouse RP, Dearholt SL, Poe SS, Pugh LC, White KM. Johns Hopkins Nursing Evidence-Based
Practice Model and Guidelines. Indianapolis, IN: Sigma Theta Tau International;
Strength of Recommendation Taxonomy (SORT). American Academy of Family Physicians
Web site. http://www.aafp.org/online/en/home/publications/journals/afp/afpsort.html.
Accessed January 9, 2013.
Understanding research study designs. University of Minnesota Bio-Medical Library
Web site. http://www.biomed.lib.umn.edu/guides/understanding-research-study-designs.
Accessed January 9, 2013.