Levels of Evidence

PLEASE NOTE: Evidence levels are only used for research involving human subjects. Basic science and animal studies should be listed as NA.

Level Therapy/Prevention or Etiology/Harm Prognosis Study Study 1a Systematic review of randomized Systematic review of prospective cohort controlled trials studies 1b Individual randomized controlled trial Individual prospective cohort study 2a Systematic review of cohort studies Systematic review of retrospective cohort studies 2b Individual cohort study Individual retrospective cohort study 2c “Outcomes research” “Outcomes research” 3a Systematic review of case-control studies 3b Individual case-control study 4 Case series (with or without comparison) Case series (with or without comparison) 5 Expert opinion Expert opinion NA Animal studies and basic research Animal studies and basic research

Case-Control Study. Case-control studies, which are always retrospective, compare those who have had an outcome or event (cases) with those who have not (controls). Cases and controls are then evaluated for exposure to various risk factors. Cases and controls generally are matched according to specific characteristics (eg, age, sex, or duration of disease).

Case Series. A case series describes characteristics of a group of patients with a particular disease or patients who had undergone a particular procedure. A case series may also involve observation of larger units such as groups of hospitals or municipalities, as well as smaller units such as laboratory samples. Case series may be used to formulate a case definition of a disease or describe the experience of an individual or institution in treating a disease or performing a type of procedure. A case series is not used to test a hypothesis because there is no comparison group.

Cohort Study. A prospective cohort study follows a group or cohort of individuals who are initially free of the outcome of interest. Individuals in a cohort generally share some underlying characteristic (age, sex, or exposure to a risk factor). Individuals who developed the outcome are compared with those who did not.

Cohort. A group of individuals who share a common exposure, experience, or characteristic, or a group of individuals followed-up or traced over time in a cohort study.

Randomized Controlled Trial. Randomized controlled trials assess efficacy of the treatment intervention in controlled, standardized, and highly monitored settings, and are usually among highly selected samples of patients.

The methods of RCTs must be described in detail to allow the reader to judge the quality of the study, replicate the study intervention, and extract pertinent information for comparison with other studies. The CONSORT statement (www.consort-statement.org) provides a checklist to help ensure complete reporting to RCTs. A flow diagram should be included to outline the flow of participants in the study, including when and why participants dropped out or were lost to follow-up and how many participants were evaluated for the study end points.

Outcomes Research. Research that investigates the outcomes of health care practices. It has been defined as the study of the end results of health services that takes patients’ experiences, preferences, and values into account—is intended to provide scientific evidence relating to decisions made by all who participate in health care.