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JAMAevidence Glossary

Terms are derived from Users' Guides to the Medical Literature: A Manual for Evidence-Based Practice, 2nd Edition, The Rational Clinical Examination: Evidence-Based Clinical Diagnosis and Care at the Close of Life: Evidence and Experience. Updated October 2014.
Download a PDF of the glossary (295 KB). (Adobe Reader is required to open or print PDF files.)



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O

Observational studies
An observational study can be used to describe many designs that are not randomized trials (eg, cohort studies or case-control studies that have a goal of establishing causation, studies of prognosis, studies of diagnostic tests, and qualitative studies). The term is most often used in the context of cohort studies and case-control studies in which patient or caregiver preference, or happenstance, determines whether a person is exposed to an intervention or putative harmful agent or behavior (in contrast to the exposure’s being under the control of the investigator, as in a randomized trial).

Observer bias
Occurs when an observer’s observations differ systematically according to participant characteristics (eg, making systematically different observations in treatment and control groups). See also Bias.

Obstructive sleep apnea (OSA)
A sleep disorder characterized by periods of interrupted breathing during sleep (apnea), shallow breaths or low respiratory rate (hypopnea), and excessive sleepiness during the day.

Odds
The ratio of events to non-events; the ratio of the number of study participants experiencing the outcome of interest to the number of study participants not experiencing the outcome of interest.

Odds ratio
A ratio of the odds of an event in an exposed group to the odds of the same event in a group that is not exposed.

Odds reduction
The odds reduction expresses, for odds, what relative risk reduction expresses for risks. Just as the relative risk reduction is 1 – relative risk, the odds reduction is 1 – relative odds (the relative odds and odds ratio being synonymous). Thus, if a treatment results in an odds ratio of 0.6 for a particular outcome, the treatment reduces the odds for that outcome by 0.4.

Odynophagia
Pain that occurs when swallowing.

Open-ended questions
Questions that offer no specific structure for the respondent’s answers and allow the respondents to answer in their own words.

Opinion leaders
See Local opinion leaders.

Opportunistic sampling
See Convenience sample.

Opportunity costs
The value of (health or other) benefits forgone in alternative uses when a resource is used.

Orthotopic liver transplantation
A type of transplantation in which the native liver is removed and the new liver is placed in the same anatomical location. A well-established treatment for end-stage liver disease and liver failure.

Osler sign
While feeling the radial pulse, occlude the brachial artery by cuff inflation or by direct pressure using the other thumb. If the radial artery remains palpable as a firm "tube," the Osler sign is positive. The test's usefulness for detecting pseudohypertension is debatable.

Osteomyelitis
Inflammation of the bone that is almost always due to infection (bacterial or mycobacterial) and frequently associated with overlying soft tissue infections such as diabetic foot ulcers.

Outcome variable
The target variable of interest. The variable that is hypothesized to depend on or be caused by another variable, the independent variable. See also Treatment target; Endpoint.

Overview
A type of review in which primary research relevant to a question is examined and summarized, and an effort is made to identify all available literature (published or unpublished) that pertains to that question.
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