1. The time interval between invasion by an infectious agent and appearance of the first sign or symptom of the disease in question. See also LATENT PERIOD.

2. In a VECTOR, the period between entry of the infectious agent into the vector and the time at which the vector becomes infective; i.e., transmission of the infectious agent from the vector to a fresh final host is possible (extrinsic incubation period).

a - exposed with disease

b - unexposed with disease

c - exposed without disease

d - unexposed without disease

The odds ratio (cross-product ratio) is ad/bc. 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 favor of exposure among noncases (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 (FORCE OF MORBIDITY) in the exposed and unexposed (no rarity assumption is required for this). The disease-odds ratio for a cohort or cross sectional study is the ratio of the odds in favor of disease among the exposed (a/c) to the odds in favor of disease among the unexposed (b/d). This reduces to ad/bc and hence is equal to the exposure-odds ratio for the cohort or cross section. 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 favor of getting disease, if exposed, to the odds in favor of getting disease if not exposed. The odds ratio derived from a cohort study is an estimate of this.

1. The ratio of the RISK of disease or death among the exposed to the risk among the unexposed; this usage is synonymous with RISK RATIO.

2. Alternatively, the ratio of the cumulative incidence rate in the exposed to the cumulative incidence rate in the unexposed, i.e., the rate ratio.

3. The term relative risk has also been used synonymously with odds ratio and, in some biostatistical articles, has been used for the ratio of FORCES OFMORBIDITY. The use of the term relative risk for several different quantities arises from the fact that for "rare" diseases (e.g., most cancers) all the quantities approximate one another. For common occurrences (e.g., neonatal mortalityin infants under 1500-g birth weight), the approximations do not hold. See also CUMULATIVE INCIDENCE RATIO; ODDS RATIO; RATE RATIO; RISK RATIO

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