George Engel promulgated a theoretical model, based on general systems theory, of the etiology of mental disease that remains central to epidemiologic investigations into the twenty-first century. Research demonstrates that unitary explanations are not adequate to explain disease etiology or thus inform appropriate prevention and treatment strategies. Engel suggested an interrelatedness among biological, psychological, and social factors. Biological factors include hereditary, anatomic, and molecular factors and those factors related to gender, age, and ethnicity. Psychological factors include temperament, personality, motivation, emotion, attention, and cognition. According to Engel’s theory, social factors included family, society, culture, and environment; other authors would include religious and spiritual as well as economic factors in this group. This model pervades epidemiology, yet recent efforts focus not on simply integrating the different elements but rather moving toward a transdisciplinary approach that breaks down traditional disciplinary barriers. From this perspective, psychiatric epidemiologists explore the frequency, distribution, outcome, and causation of psychiatric disorders.
Identifying a case becomes the first task of the epidemiologist. An essential component of all these uses is the determination of valid denominators to compare the characteristics of populations with and without disease. For example, to determine the prevalence of a case (i.e., the frequency of the case in a given population at a given point in time), one must know the number of persons both with and without the disorder in the population. To determine the incidence of a case (the number of new cases that emerge in the population over a given interval—usually 1 year), one must know the number of people in the population at the beginning of an interval who do not experience the disorder. A list of key terms in epidemiology can be found in Table 2–1.
Table 2–1 Key Terms in Epidemiology
Prevalence: The frequency of a given disorder in a population at a particular point of time (i.e., it is the ratio of the number of cases of a disorder in the population divided by the number of persons in the population). Although community surveys take time to complete (usually 3 months to 1 year), it is assumed that the results of such studies estimate the frequency of a disorder within a population (usually given as a percentage) on a given day. In some cases, prevalence is measured not as the frequency on a given day but as the frequency of all cases of the disorder that are present during some interval of time, such as 1 month or 1 year. Denominator: In the prevalence ratio, the denominator is the number of persons in the population. This number becomes important because the prevalence may vary depending on the population from which cases are selected. For example, the denominator may be all persons in a community, all females in a community, all persons attending a clinic, all persons 65 years of age or older attending a clinic, or all African Americans in the community. For each denominator, prevalence may vary. Incidence: The likelihood, over a period of time (usually 1 year), that an individual who is free from a given disorder will develop it. For example, if 1000 persons are free of a disorder on January 1 and 100 develop the disorder during the next 12 months, then the incidence of that disorder is 10%. In most cases, incidence is much lower than prevalence, and, naturally, because incidence reflects a rate, the collection of data at two or more points in time is necessary to determine incidence. Risk: The likelihood that an individual will experience a psychiatric disorder. In this sense, risk is basically a measure of incidence. Risk factor: Any factor that may increase the likelihood that a person will develop a psychiatric disorder. For example, it is known that female gender, younger age, lower socioeconomic status, and being divorced are risk factors for developing major depression. A risk factor may not necessarily be causal. Risk profile: The array of risk factors associated with a specific disorder. Relative risk: The increased (or decreased) risk for developing a disorder among persons with a risk factor, compared to persons without a risk factor. Comorbidity: The presence of at least two distinct disorders in the same individual, each with its own etiology, presentation, and course. |
In recent years, psychiatric epidemiologists have recognized that cases often occur simultaneously in the same person (the cases are comorbid). Recent studies have emphasized not only the prevalence and incidence of individual cases but also comorbid cases.
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