EPI2-7: Incidence Rate Defined

INCIDENCE RATE:

In some studies different subgroups of the study population are considered at risk of getting the disease during different periods of time and individuals are considered at risk for periods of varying lengths.

This variation in the risk period derives from the fact that different individuals enter the study at different points in time or that some migrate out during the observation period.

In such situations the incidence proportion will not be directly calculable from the data. In these cases it will be necessary to use the basic measure of disease occurrence, which is the incidence rate (IR). It is also known as incidence density, and it is defined as:

The sum of the time periods in the denominator is often measured in years and is referred to as person-years, person-time, or risk time. For each individual in the population the time at risk is the time during which that individual is in the study population and remains free from the disease, and therefore at risk to get it. These time periods at risk are then summed for all individuals.

The rationale is that the total number of individuals who move from the disease-free state to the disease state during any period of time is the product of three factors: size of the population, the length of the time period of observation, and the "force of morbidity" that operates on the population. It is this "force of morbidity" that the incidence rate measures. Therefore the incidence rate is obtained by dividing the number of cases by the person-years at risk, which is the sum of the individual periods of observation time at risk for each individual in the population. By dividing the number of cases by the time at risk, the length of the observation period is taken into account. Also, individuals entering or exiting the population during the observation period because of migration, competing mortality, or any other reason are automatically accounted for. Thus, by including time at risk in the definition, the incidence rate accounts for the major drawbacks encountered with the incidence proportion measure, namely, averaging over the whole time period and removal of individuals who may have become cases by competing causes of mortality or migration.

In practical situations it is often not possible to calculate the time at risk for each individual. It may not even be possible to exclude the period during which some of the individuals are no longer at risk because they already have developed the disease. An approximation to the total time at risk that is usually satisfactory can be obtained by multiplying the average of the population size at the beginning and the end of the observation period by the length of the period. The size of the population at the middle of the observation period may also be used for this approximation. This is what happens for annually reported rates such as the mortality or birth rates. The size of the population at mid-year is estimated as the denominator.

The incidence rate is not a proportion like the two previous measures, since the numerator is the number of cases and the denominator is the number of person-time units. The magnitude of the incidence rate can never be less than zero but there is no upper limit for the numerical value of the incidence rate. This property can easily be understood by considering that the value of the incidence rate would change drastically if person years were used instead of person months. In this case the incidence rate would be 12 times higher when person years are used in the denominator.

General Introduction to Occupational Health: Occupational Hygiene, Epidemiology & Biostatistics by Prof Jonny Myers is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License