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Effect or effect size: A statistical estimate of the effect of an intervention or treatment in a study that is used to determine samples sizes, compare treatments, and combine results across studies in meta-analysis.22 Values can be negative or positive and have no units. Values greater than 0.8 are uncommon and indicate a significant impact of a treatment or intervention.

Effective Health Care (EHC) Program: Funds individual researchers, research centers, and academic organizations to work together with the Agency for Healthcare Research and Quality to produce effectiveness and comparative effectiveness research for clinicians, consumers, and policymakers.23 The EHC Program: 1) reviews and synthesizes published and unpublished scientific evidence; 2) generates new scientific evidence and analytic tools; and 3) synthesizes research findings and/or generates and translates them into useful formats for various audiences. The EHC Program has three primary products: 1) research reviews: (comparative effectiveness reviews and technical briefs): 2) research reports;and 3) summary guides. Website:

Effectiveness: The extent to which an intervention works under real-world conditions (i.e., in practice).24 Effectiveness studies involving drugs examine whether they work when they are used the way that most individuals take them. A treatment is effective when most individuals who have the disease would improve if they used the treatment. Effectiveness studies ask the question, “Does it work?” Clinical trials that assess effectiveness are sometimes called pragmatic trials, management trials, or practical trials.

Effectiveness review: Comprehensive reports based on available evidence that evaluate the effectiveness of interventions.15 They are similar to comparative effectiveness reviews except there may not be a clear comparator for interventions evaluated in effectiveness reviews. Evidence-based Practice Centers develop effectiveness reviews with funding through the Agency for Healthcare Research and Quality’s Effective Health Care (EHC) Program. The other type of research review produced by the EHC Program is a technical brief.

Efficacy: The extent to which an intervention produces a beneficial result under ideal conditions (i.e., in clinical trials).6, 24 Efficacy trials ask the question, “Can it work?” Clinical trials that assess efficacy are called explanatory trials.

EHC Program: See Effective Health Care Program.

Eisenberg Center: The John M. Eisenberg Center for Clinical Decisions and Communications Science translates comparative effectiveness reviews and research reports created by the Agency for Healthcare Research and Quality’s Effective Health Care Program into short, easy-to-read guides and tools for use by consumers, clinicians, and policymakers.25

Endpoint: See outcome.

EQUATOR: Acronym for Enhancing the QUAlity and Transparency Of health Research.26 The EQUATOR Network was launched to coordinate initiatives to promote transparent and accurate reporting of health research and to assist in the development of reporting guidelines. Website:

Estimate of effect: See treatment effect.

Evidence synthesis: The collation, combination, and summary of findings from a body of evidence.11 Can be qualitative or quantitative (meta-analysis). See also systematic review and meta-analysis.

Evidence-based medicine: Conscientious, judicious use of current best scientific evidence in making decisions about patient care.27

Evidence-based Practice Centers (EPCs): The Agency for Healthcare Research and Quality created the EPCs in 1997 to conduct research reviews for the Effective Health Care (EHC) Program.28 The EPCs are located at medical schools, universities, or medical centers throughout the country. The EPCs produce comparative effectiveness reviews or effectiveness reviews on medications, devices, and other health care services with the goal of helping patients, physicians, and policymakers make better decisions about treatments.

Experiment: See experimental study.

Experimental group: Participants in the experimental arm of a study that receive the intervention of interest. Also called treatment group.

Experimental intervention: An intervention under evaluation.6 In a controlled trial, an experimental intervention arm is compared with one or more control arms, and possibly with additional experimental intervention arms.

Experimental study: A study in which the investigators actively intervene to test a hypothesis.11 It is called a trial or clinical trial when human participants are involved.4 See also controlled trial.

Explanatory trial: A controlled trial that seeks to measure the benefits of an intervention in an ideal setting (efficacy) by testing causal research hypotheses with the aim of understanding.29, 30 Trials of health care interventions are often described as either explanatory or pragmatic. See also pragmatic trial.

External validity: The extent to which results provide a correct basis for generalizations to other circumstances (e.g., populations, settings).6 Also called generalizability, applicability. See also applicability.