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Footnotes

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Footnotes to Epidemiology Back to MedAnth 101  
Adjusted (or standardized) rates are fictitious summary rates constructed to permit fair comparison between groups differing on some important characteristic. For example, rates are commonly age-adjusted. Top of the page

Crude rates are summary rates based on the actual number of events in population over a given period of time. Top of the page

Incidence rate = number of new cases of a disease over a time period divided by the population at risk during that time period. Incidence rate is a measure of the rate at which people without a disease develop the disease during a specified period of time. [Compare prevalence rate] Top of the page

Prevalence rate = number of existing cases of a disease at a point in time divided by population at risk at that point in time. Prevalence rate measures the proportion of the people in the population who have the disease at a given point in time. [Compare incidence rate] Top of the page

Case fatality rate = number of deaths from a disease divided by the number of cases of that disease (usually expressed as a percentage). Case fatality rate indicates the probability of death among diagnosed cases. Top of the page

Endemic occurrence is defined as the "habitual presence of a disease or infectious agent within a geographic area… or the usual prevalence of a given disease within such area. Endemic describes levels of infection that do not exhibit wide fluctuations through time in a defined place. [Compare epidemic] For microparasites like measles, the term is used slightly differently to indicate an infection which can persist in a population in the long term without needing to be reintroduced from outside. Stable endemicity is where the incidence of infection or disease shows no secular trend for increase or decrease. Top of the page

Epidemic: "the occurrence in a community or region of a [temporal] grouping of illnesses… of similar nature, clearly in excess of normal expectancy." [Compare endemic.] Top of the page

Epidemiology is the study of the distribution and determinants of diseases and injuries in human populations. Top of the page

Holoendemic is an infection whose prevalence is fairly uniform throughout a region, country or continent. Often used in the malaria literature. Top of the page

Pandemic is an epidemic widely distributed in space. Top of the page

Pathogenicity = cases of disease divided by total number infected by the disease.  Pathogenicity refers to the capacity of an agent to cause disease in an infected host. [Compare virulence.] Top of the page

Sensitivity versus Specificity (click here)

Specific rates are summary rates based on the actual number of events in specific population over a given period of time. Top of the page

Specificity versus Sensitivity (click here)

Virulence = cases of severe and fatal disease divided by all cases of the disease. Virulence refers to the ability of an agent to produce serious illness. [Compare pathogenicity.] Top of the page
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