# Epidemiologic Data Measurements

2 Epidemiologic Data Measurements

# I Frequency

## C Illustration of Morbidity Concepts

The concepts of incidence (incident cases), point prevalence (prevalent cases), and period prevalence are illustrated in Figure 2-2, based on a method devised in 1957.1 Figure 2-2 provides data concerning eight persons who have a given disease in a defined population in which there is no emigration or immigration. Each person is assigned a case number (case no. 1 through case no. 8). A line begins when a person becomes ill and ends when that person either recovers or dies. The symbol t1 signifies the beginning of the study period (e.g., a calendar year) and t2 signifies the end.

In case no. 1, the patient was already ill when the year began and was still alive and ill when it ended. In case nos. 2, 6, and 8, the patients were already ill when the year began, but recovered or died during the year. In case nos. 3 and 5, the patients became ill during the year and were still alive and ill when the year ended. In case nos. 4 and 7, the patients became ill during the year and either recovered or died during the year. On the basis of Figure 2-2, the following calculations can be made. There were four incident cases during the year (case nos. 3, 4, 5, and 7). The point prevalence at t1 was four (the prevalent cases were nos. 1, 2, 6, and 8). The point prevalence at t2 was three (case nos. 1, 3, and 5). The period prevalence is equal to the point prevalence at t1 plus the incidence between t1 and t2, or in this example, 4 + 4 = 8. Although a person can be an incident case only once, he or she could be considered a prevalent case at many points in time, including the beginning and end of the study period (as with case no. 1).

## D Relationship between Incidence and Prevalence

Figure 2-1 provides data from the U.S. Centers for Disease Control and Prevention (CDC) to illustrate the complex relationship between incidence and prevalence. It uses the example of AIDS in the United States from 1981, when it was first recognized, through 1992, after which the definition of AIDS underwent a major change. Because AIDS is a clinical syndrome, the present discussion addresses the prevalence of AIDS, rather than the prevalence of its causal agent, human immunodeficiency virus (HIV) infection.

In Figure 2-1, the full height of each year’s bar shows the total number of new AIDS cases reported to the CDC for that year. The darkened part of each bar shows the number of people in whom AIDS was diagnosed in that year, and who were known to be dead by December 31, 1992. The clear space in each bar represents the number of people in whom AIDS was diagnosed in that year, and who presumably were still alive on December 31, 1992. The sum of the clear areas represents the prevalent cases of AIDS as of the last day of 1992. Of the people in whom AIDS was diagnosed between 1990 and 1992 and who had had the condition for a relatively short time, a fairly high proportion were still alive at the cutoff date. Their survival resulted from the recency of their infection and from improved treatment. However, almost all people in whom AIDS was diagnosed during the first 6 years of the epidemic had died by that date.

The total number of cases of an epidemic disease reported over time is its cumulative incidence. According to the CDC, the cumulative incidence of AIDS in the United States through December 31, 1991, was 206,392, and the number known to have died was 133,232.2 At the close of 1991, there were 73,160 prevalent cases of AIDS (206,392 − 133,232). If these people with AIDS died in subsequent years, they would be removed from the category of prevalent cases.

On January 1, 1993, the CDC made a major change in the criteria for defining AIDS. A backlog of patients whose disease manifestations met the new criteria was included in the counts for the first time in 1993, and this resulted in a sudden, huge spike in the number of reported AIDS cases (Fig. 2-3). Because of this change in criteria and reporting, the more recent AIDS data are not as satisfactory as the older data for illustrating the relationship between incidence and prevalence. Nevertheless, Figure 2-3 provides a vivid illustration of the importance of a consistent definition of a disease in making accurate comparisons of trends in rates over time.

This conceptual formula works only if the incidence of the disease and its duration in individuals are stable for an extended time. The formula implies that the prevalence of a disease can increase as a result of an increase in the following:

Aug 27, 2016 | Posted by in PUBLIC HEALTH AND EPIDEMIOLOGY | Comments Off on Epidemiologic Data Measurements

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