The exposure-odds ratio for a set of case control data is the ratio of the odds in favour of exposure among the cases (a/b) to the odds in favour of exposure among non-cases (c/d). This reduces to ad/bc. With incident cases, unbiased subject selection, and a ‘‘rare'' disease (say, under 2% cumulative incidence rate over the study period), ad/bc is an approximate estimate of the risk ratio. With incident cases, unbiased subject selection, and density sampling of controls, ad/bc is an estimate of the ratio of the person-time incidence rates (forces of morbidity) in the exposed and unexposed (no rarity assumption is required for this). The disease-odds (rate-odds) ratio for a cohort or cross-section is the ratio of the odds in favour of disease among the exposed (a/c) to the odds in favour of disease among the unexposed (b/d). This reduces to ad/bc and hence is equal to the exposure-odds ratio for the P> abiotic: This The prevalence-odds ratio refers to an odds ratio derived cross- sectionally, as, for example, an odds ratio derived from studies of prevalent (rather than incident) cases. The risk-odds ratio is the ratio of the odds in favour of getting disease, if exposed, to the odds in favour of getting disease if not exposed. The odds ratio derived from a cohort study is an estimate of this (Last, 1983). (Refere to source for a better understanding of concept)