Incidence counts new disease diagnoses during a defined time period.
The difference between incidence and prevalence is a fundamental distinction in epidemiology.
Incident sampling of both cases and controls is preferable to prevalent sampling
Terminology is not standardized and is used carelessly even by those who know better.
Key to understanding measures is to pay attention to how these 3 elements are used:
Szklo's table below summarizes the assumptions made for both types of analysis.
The denominator for incidence does not have to be a count of individual persons as one can see in the following table.
Table 2. Different authors terminology of incidence measures (Tapia 1997).
Incidence measured as | ||||
Author | Count of events | Events per time unit | Events per element-time unit | Probability |
---|---|---|---|---|
E | E/T | E/NT | E/N | |
Last | Incidence | Person-years incidence rate | Cumulative incidence rate | |
Freeman | Incidence rate | Incidence density or force of morbidity | ||
Kelsey | Incidence rate | Risk of probability of developing disease; cumulative incidence | ||
Morgenstern | (Instantaneous) rate | "Relative rate"; (relative incidence rate) | Risk (of incidence) | |
Rothman | Incidence Rate | Cumulative incidence | ||
Selvin | Incidence rate | Incidence proportion | ||
Haberman1 Haberman2 | Incidence rate | Incidence rate |
As stated above there is a lack of a standard vocabulary for measures of incidence even among epidemiological text books. If you look at the chart above, we have added the E, N, and T to this table in order to show how focusing on those 3 elements can clarify what is being measured despite the differences in terminology among these authors.
Note, however, that when events per person-time unit is being measured, most authors call this a incidence rate (with one of these author calling it an incidence density).
Numerator is always the number of new events in a time period (E)
Examine the denominator (persons or person-time) to determine the type of incidence measure
Looking at whether the denominator is a number of persons or the product of persons times time will tell you whether you are looking at cumulative incidence or at an incidence rate.
Freeman, J., & Hutchison, G. B. (1980). Prevalence, incidence and duration. Am J Epidemiol, 112 (5), 707-723.
Haberman, S. (1978a). Mathematical treatment of the incidence and prevalence of disease. Soc Sci Med, 12, 147 – 152.
Haberman, S. (1978b). Probalistic treatment of the incidence and prevalence of disease. Soc Sci Med, 12, 159 – 161.
Kelsey, J., Douglas, T., & Evans, A. (1980). Methods in Observational Epidemiology. New York: Oxford University Press.
Last, J. (1995). A Dictionary of Epidemiology (3rd ed.). New York: Oxford University Press.
Morgenstern, H., Kleinbaum, D. G., & Kupper, L. L. (1980). Department of Epidemiology and Public Health, School of Medicine, Yale University, New Haven, CT. Int J Epidemiol, 9(1), 97-104.
Rothman, K. (1986). Modern Epidemiology. Boston: Little, Brown.
Selvin, S. (1991). Statistical Analysis of Epidemiologic Data. New York: Oxford University Press
Szklo, M., & Nieto, F. (2007). Epidemiology: Beyond the Basics (2nd Edition ed.). Boston: Jones and Bartlett Publishers.
Tapia Granados, J. A. (1997). On the terminology and dimensions of incidence. J Clin Epidemiol, 50 (8), 891-897.