A rate is a measure of change in one quantity with respect to change in another. As used in epidemiology, this typically refers to an incidence rate, where the numerator is the number of new events and the denominator is total person-time at risk. This is one of the key measures of occurrence of disease in populations and gives an estimate of how fast disease or death is happening in a given population.

An example calculation of an incidence rate can be done using data from Table 1. A total of three events occurred, and the total person-time at risk summed over all population members is 30 + 17 + 22 + 11 + 20 =100 person-years, giving a rate of 3/100 or 0.03 per year. ...

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