Hierarchy of evidence

A hierarchy of evidence is a method of categorizing the quality of potential evidence, usually for use in a medical study or making decisions in health care. Experts in evidence-based medicine gather facts and other information to support their ideas and actions but must rank this evidence and focus mainly on the highest-quality material. Different experts have rendered hierarchy charts in different ways. However, a common form is a triangle with seven levels. Weaker evidence occupies the broader bottom levels. Stronger evidence occupies the narrower top levels. Health-care experts start with the best evidence and only move to lower levels if high-quality evidence is lacking.

Background

Evidence is a collection of factual information that supports the truth or validity of a particular claim or idea. Evidence is a crucial part of many fields and occupations in which opinions, beliefs, traditions, and intuition are not sufficient as the sole basis for important decisions. During the twentieth century, evidence became increasingly important in many areas of life, including medicine, public policy, law, and education. The term “evidence-based practice” arose around the 1990s in reference to this emphasized focus on evidence.

One of the foundational branches of evidence-based practice is evidence-based medicine (EBM). This approach to health care places a strong emphasis on the use of current scientific data to better care for patients. EBM practitioners combine all the evidence they can gather, along with their own experiences and expertise and the needs and values of the patient, to make decisions. They use a wide range of analyses, including analyzing potential risks versus benefits and the likely results of decisions. They also consult the latest medical reports and other literature to learn about new evidence that they can translate into their own work.

The field of EBM may reach across multiple disciplines to gather the evidence necessary. Some fields commonly considered by EBM practitioners include engineering, statistics, and epidemiology. Epidemiology is the analysis of the causes, patterns, and distribution of health conditions among a given group of people. Modern medical experts have embraced this study as a cornerstone of preventive medicine. Basically, by examining the frequency and occurrences of a disease in a population, experts can better understand the nature of the illness and suggest methods for people in that population to avoid becoming ill.

Overview

In modern health care, evidence is a crucial component of all major decisions. Health experts may perform studies or analyze reports to gain the information they need to make sound choices about their patients’ well-being. Potentially useful evidence may take many forms and be found in countless places. However, not all this evidence may be equally useful. Potential evidence might be poorly supported, such as a poorly documented experiment that was not replicated. The evidence might be overly limited in scope, or biased toward a particular perspective. In some cases, potential evidence may turn out to be completely inaccurate, and using it would likely bring more harm than benefit to patients.

For this reason, practitioners of EBM have adopted heuristics, investigative, problem-solving, and decision-making methods. Heuristics apply practical approaches to problems with the goal of reaching an estimate or short-term goal in a relatively quick and easy manner, even if the solution reached is not certain to be perfect and may need revision or replacement later. One of the most important modern heuristics in EBM is called a hierarchy of evidence.

A hierarchy of evidence is a method of ranking available evidence, from the weakest to the strongest. Although dozens of variations of this hierarchy exist that were created by experts in various fields, this form of heuristic is characterized by a desire to focus attention on the best sources of evidence and reduce emphasis on less-reliable or less-useful sources. These hierarchies are also similar in that they are usually styled as a triangular shape. The large body of less-desirable evidence makes up the broad flat bottom of the triangle, with each subsequent higher level containing a smaller amount of better information. The highest-quality evidence occupies the tip, the smallest and highest part of the triangle.

One of the most widely adopted versions of the hierarchy of evidence has seven levels. The base level contains background information and expert opinions. Experts may access this information in textbooks, encyclopedias, and similar reference guides. This information may help to formulate ideas and guide the most general decision-making but remains relatively vague and poorly supported compared to better sources of evidence. In addition, this evidence normally takes years to be published, so it is unlikely to contain cutting-edge material.

The next three levels, in ascending order, include case-controlled studies; cohort studies; and randomized controlled trials (RCTs). Case-control studies compare patients with certain health concerns or procedures to controls (patients who have not had those concerns or procedures). Cohort studies are more in-depth versions that use groups of patients and, typically, more detailed and long-lasting analysis. RCTs are studies that involve randomized groups of patients who are carefully studied and documented over time. These three sources of evidence are of better quality than the lowest level, yet remain “unfiltered.” In the sciences, unfiltered evidence is information that has not been critically reviewed by expert peers.

The top three levels represent the best forms of information. These sources have been filtered, or critiqued by peers. In ascending order, the levels are critically appraised articles; critically appraised topics; and systematic reviews. Articles refer to single studies that have been carefully reviewed. Topics refer to groups of articles or other studies on a related topic that have been reviewed and deemed to represent a valid overall idea. The highest level is occupied by systematic reviews, which include in-depth studies, evaluations, and summaries of all available information on a given topic.

Bibliography

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