Introduction

Evidence-based medicine (EBM) or Evidence-Based Practice (EBP) is the process of systematically reviewing, appraising, and using clinical research findings to support the delivery of clinical care to patients. This practice seeks to prioritize information in a hierarchy of evidence by study design from the most biased to the least biased EBM is a structured approach to literature evaluation, leading to decisions which are based upon probability. The true effect of a medical intervention is never known for certain; thus, medical professionals can only estimate where the truth may lie. In other words, the better the study, the closer one moves towards obtaining accuracy and truth regarding the patient and medical intervention.

Doctors and other medical professionals use evidence to make a variety of decisions regarding patient care. The majority of clinical questions, however, yield an often overwhelming amount of information as well as problematic or contradictory conclusions. One approach to making clinical decisions is EBM. Under this process, doctors define the clinical question, gather the evidence, appraise the evidence, and apply the evidence while taking into account their experience and patients’ particular circumstances. EBM generally follows five levels of evidence in order to deliver the most effective care to patients.

Levels of Evidence

Levels of evidence in EBM are based upon study design, research, synthesis and analysis. An understanding of how various levels of evidence are reported and how this literature is organized will assist the searcher in obtaining the highest levels of evidence for a particular clinical question. High levels of evidence may not exist for all clinical questions because of the nature of medical problems as well as research and ethical limitations. Levels of evidence provide specific methods in order to sort through and rate the quality of the available medical literature. BBM studies contain different purposes and procedures. The most common study purpose is evaluation of therapy; however, others include studies and analysis of diagnosis and prognosis.

Level I

Level I Evidence narrows the field of research to high-quality randomized clinical trials, systematic reviews and meta-analysis. Randomized controlled trials are true controlled experiments. In these types of trials, two or more groups of subjects receive different interventions, are followed and observed over time and then are compared.  Systematic reviews are a specific type of review article which can be considered level I evidence when randomized controlled trials are utilized. Systematic reviews, unlike textbooks or narrative reviews, require meticulous planning and methods which minimize bias and error. Meta-analysis is a specialized type of systematic review which pool data for a quantitative rather than a qualitative result.

Level II

Cohort studies are a type of observational comparative study which begins with an exposure instead of an outcome. This prospective study minimizes recall bias. Cohort studies are expensive and time-consuming and require attention to detail in control group evaluation. Results typically are expressed as a relative risk rather than odds ratios.

Level III

Case-control studies are a type of observational study in which the outcome has already occurred. In contrast to Level II Cohort studies, case-control studies are retrospective studies which utilize a comparator. Case control studies are known to produce the most effective evaluation of rare conditions. The results of these studies are usually expressed as odds ratios.

Level IV

This level comprises case reports and series, which are extremely pervasive in medical literature. These retrospective reports are easy to compile and relatively inexpensive.  Case studies and case series are best utilized in order to develop hypotheses rather than test hypotheses.

Level V

Unsystematic anecdotal information is considered the least valid form of evidence regarding therapeutic and/or medical interventions. This type of information is most effective when used to develop hypotheses which are tested using more rigorous study designs. Anecdotal information is highly biased and nearly impossible to verify. The sources of anecdotal information may include colleagues, experts in the field, and drug and device manufacturers.

Effects in Utilization of Evidence Levels

EBM Evidence Levels enable practitioners to become more familiar with different research methods and designs. Research studies have the potential for continuous improvement as doctors perform higher quality studies. In addition, EBM professionals will have the opportunity to monitor publication trends in their respective areas of clinical research. EBM practitioners have cited strengths of utilizing EBM Evidence Levels, as these levels are valuable the formulation and justification of medical professionals’ presentation of factual and statistically supported decisions (as opposed to opinions).

Additional Resources

Users’ Guides to Evidence-Based Practice
This resource includes the complete set of EBM Users’ Guides originally published as a series in the Journal of the American Medical Association (JAMA).

Evidence-Based Medicine Tutorial
This comprehensive and informative tutorial is put together by State University of New York (SUNY) Medical Center.

Centre for Evidence-Based Medicine (Oxford University)
The Centre promotes evidence-based health care and provide support and resources to anyone who wants to make use of them.

Introduction to Evidence-Based Medicine
Compiled by Duke University Medical Center Library and University of North Carolina at Chapel Hill Health Sciences Library, this comprehensive guide to Evidence-Based Medicine covers a variety of educational and research related topics.

Glossary of Terms
This glossary outlines the key terms and concepts as utilized by EBM practitioners.

EBM Lecture
This ten minute lecture provides history, definitions, and general background information for students as well as beginning and advanced EBM practitioners.

National Guidelines Clearinghouse
This resource contains a public database for EBP guidelines.