The Federal Register's definition of herd incidence rate, reproduced below, is, we believe, incorrect. A rate is not a proportion. The herd incidence rate as defined is actually the period prevalence. A herd, flock or other group of animals would be counted only once during the specified time period regardless of the number of cases that occur in the group of animals during the period. It is usually expressed in relation to the population at risk.
Incidence rate is the usual measure of risk and is calculated per week, per year, and so on.
The OIE defines incidence as meaning the number of new cases or outbreaks within a specified period of time in a defined animal population'. It is commonly expressed as point prevalence, that is, the amount of disease in a population at a particular point in time. Although prevalence can be defined simply as the number of affected animals, it is usually expressed in terms of the number of diseased animals in relation to the number of animals in the population at risk of developing the disease. This is a proportion that represents the probability of an animal having a specified disease at a given time. Prevalence can take values between 0 and 1 and is dimensionless.