O

Observational study
A study in which the investigators do not seek to intervene, and simply observe the course of events. Changes or differences in one characteristic (e.g. whether or not people received the intervention of interest) are studied in relation to changes or differences in other characteristic(s) (e.g. whether or not they died), without action by the investigator.  There is a greater risk of selection bias than in experimental studies. See also randomised controlled trial. (Also called non-experimental study.)

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. For example, if in a sample of 100, 20 people died and 80 people survived the odds of death are 20/80 = 1⁄4, 0.25 or 1:4.

Odds ratio (OR)
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 one indicates no difference between comparison groups. For undesirable outcomes an OR that is less than one indicates that the  intervention was effective in reducing the risk of that outcome.  When the risk is small, odds ratios are very similar to risk ratios. (Also called OR.)

Open clinical trial
There are at least three possible meanings for this term:

1. A clinical trial in which the investigator and participant are aware which intervention is being used for which participant (i.e. not blinded). Random allocation may or may not be used in such trials. Sometimes called an ‘open label’ design.
2. A clinical trial in which the investigator decides which intervention is to be used (non-random allocation). This is sometimes called an open label design (but some trials which are said to be ‘open label’, are randomised).
3. A clinical trial that uses an open sequential design.

Open sequential design
A sequential trial where the decision to stop the trial rests on the size of effect in those studies, and there is no finite maximum number of participants in the study.

OR
See odds ratios

Ordinal data
Data that are classified into more than two categories where there is a natural order to the categories; for example, non-smokers, ex-smokers, light smokers and heavy smokers. Ordinal data are often reduced to two categories to simplify analysis and presentation, which may result in a considerable loss of information.

Outcomes
A component of a participant's clinical and functional status after an intervention has been applied, that is used to assess the effectiveness of an intervention. See also primary outcome, secondary outcome.

Overview, systematic
See systematic review.