A very common aim of epidemiological investigation is to estimate the levels of disease in a population. This is of particular importance in the case of [[Surveillance and monitoring of disease|surveillance and disease monitoring systems]], and is commonly the central aim of many descriptive studies.
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A very common aim of epidemiological investigation is to estimate the levels of disease in a population. This is of particular importance in the case of [[Surveillance and monitoring of disease|surveillance and disease monitoring systems]], and is commonly the central aim of many [[Descriptive epidemiological studies|descriptive studies]]. There are two main measures of disease frequency - the '''prevalence''' and the '''incidence''' of disease, which each measure different aspects of disease. The '''time to event''' is another measure commonly used, and is closely related to the '''incidence'''.