(Science: epidemiology) A comparison of the presence of a risk factor for disease in a sample of diseased subjects and non diseased controls. The number of people with disease who were exposed to a risk factor ( I.e. ) over those with disease who were not exposed ( Io ) divided by those without disease who were exposed ( ne ) over those without who were not exposed ( no ). Thus OR=(I.e./Io)/(Ne/No)= I.e. No / Io Ne. This measure should be used for case control studies where we retrospectively look at risks in those with and without disease. Also known as exposure odds ratio.