The ratio of two probabilities. In epidemiological case-control studies it is an estimate of the relative risk. The ratio of the probability, for example, of having a disease in a population exposed to certain risk factors and the probability of having that disease in another population not so exposed, or the probability that one treatment is more effective than another. It is calculated thus: the number of individuals with disease who were exposed to a risk factor (De) over those with disease who were not exposed (Dn) divided by those without disease who were exposed (He) over those without who were not exposed (Hn).
Thus OR = (De/Dn)/( He/Hn) = DeHn/DnHe.
- synonymCross-product Ratio (written as odds ratio)