In a study of treatment effects, the odds of an outcome in a treatment comparison group is a measure of how likely it was that people in the group would experience the outcome.
The odds is the number of people in the group who experienced the outcome divided by the number of people who did not experience it.
For example, if 50 people died and 50 people did not die, the odds of dying in that group was 50/50 = 1. This is different from the probability of dying, which is the number of people experiencing the outcome divided by the total number of people in the group. In this example, the probability would be 50/100 = 0.50 or 50%.
If you feel that this definition hasn't helped you to understand the term, click on our monkey to let us know.