2 Epidemiologic Data Measurements
I Frequency
A Incidence (Incident Cases)

Figure 2-1 Incident cases of acquired immunodeficiency syndrome in United States, by year of report, 1981-1992.
The full height of a bar represents the number of incident cases of AIDS in a given year. The darkened portion of a bar represents the number of patients in whom AIDS was diagnosed in a given year, but who were known to be dead by the end of 1992. The clear portion represents the number of patients who had AIDS diagnosed in a given year and were still living at the end of 1992. Statistics include cases from Guam, Puerto Rico, the U.S. Pacific Islands, and the U.S. Virgin Islands.
(From Centers for Disease Control and Prevention: Summary of notifiable diseases—United States, 1992. MMWR 41:55, 1993.)
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.

Figure 2-2 Illustration of several concepts in morbidity.
Lines indicate when eight persons became ill (start of a line) and when they recovered or died (end of a line) between the beginning of a year (t1) and the end of the same year (± t2). Each person is assigned a case number, which is circled in this figure. Point prevalence: t1 = 4 and t2 = 3; period prevalence = 8.
(Based on Dorn HF: A classification system for morbidity concepts. Public Health Rep 72:1043–1048, 1957.)
D Relationship between Incidence and Prevalence
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.

Figure 2-3 Incident cases of AIDS in United States, by quarter of report, 1987-1999.
Statistics include cases from Guam, Puerto Rico, the U.S. Pacific Islands, and the U.S. Virgin Islands. On January 1, 1993, the CDC changed the criteria for defining AIDS. The expansion of the surveillance case definition resulted in a huge spike in the number of reported cases.
(From Centers for Disease Control and Prevention: Summary of notifiable diseases—United States, 1998. MMWR 47:20, 1999.)
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:
II Risk
III Rates
B Relationship between Risk and Rate

Figure 2-5 Circumstances under which the concept of rate is superior to the concept of risk.
Assume that populations A, B, and C are three different populations of the same size; that 10% of each population died in a given year; and that most of the deaths in population A occurred early in the year, most of the deaths in population B occurred late in the year, and the deaths in population C were evenly distributed throughout the year. In all three populations, the risk of death would be the same—10%—even though the patterns of death differed greatly. The rate of death, which is calculated using the midyear population as the denominator, would be the highest in population A, the lowest in population B, and intermediate in population C, reflecting the relative magnitude of the force of mortality in the three populations.
Rates are often used to estimate risk. A rate is a good approximation of risk if the:
Event in the numerator occurs only once per individual during the study interval.
Proportion of the population affected by the event is small (e.g., <5%).