Observational study: A study in which investigators observe the course of events and do not assign participants to the intervention.6, 11 Also called non-experimental study.

Odds ratio (OR): An estimate of the relative risk calculated in case-control studies.4, 6 It is the ratio of the odds of an event in one group to the odds of an event in another group. In studies of treatment effect, the odds in the treatment group are usually divided by the odds in the control group. An odds ratio of 1 indicates no difference between comparison groups. For undesirable outcomes, an OR of < 1 indicates that the intervention was effective in reducing the risk of that outcome.

Odds: A way of expressing the chance of an event, calculated by dividing the number of individuals in a sample who experienced the event by the number for whom it did not occur.6 For example, if in a sample of 100, 20 individuals died and 80 individuals survived, the odds of death are 20 / 80 = 1/4, 0.25 or 1:4.

Outcome: The result of an experimental study that is used to assess the effect of an intervention.4, 6  Also called endpoint.

Outcomes research: Evaluation of the effect of health care interventions on patient-related, clinical, humanistic, and economic outcomes.1