Scott Henderson Introduction
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Epidemiology deals with overall patterns. On the one hand, people are unique with their own genetic endowment and life experiences. This idiographic paradigm is balanced by the nomothetic in which recurrent and predictable patterns are sought in the whole of humankind. It is the business of psychiatric epidemiology to determine the distribution of mental disorders in populations, the factors determining that distribution, and measures that may help in their prevention.
From their undergraduate years onward, clinicians are accustomed to patients who have a disorder and who at the same time give a history of certain experiences dating from birth to their present. It may be tempting for both patient and doctor to make causal links. But if the principles of epidemiology are brought into play, some questions need to be asked first. What proportion of the general population have had the same experiences but not developed the disorder? What proportion have the same disorder but have not had these experiences? What proportion have the same disorder but are not known to health services? And could having the exposure itself be linked in some way with being known, or not known, to health services? These are the questions that are prompted by a simple two-by-two table (Table 1).
The rows are made up of persons in a population who have or do not have a particular disorder. The columns are the numbers who have or have not had a certain exposure. That exposure is being considered as a putative risk factor. It may be biological or psychosocial and may have taken place at any time from conception to the present. In every one of these questions, the clinician's traditional perspective is expanded to consider the whole population and not only the sick who have reached clinics or hospitals.
To establish a causal link between some phenomenon and a disorder is a demanding but most engaging exercise. It is well worth reading the classic expositions by
Bradford Hill (1.) and Mervyn Susser(23) on how a cause can reasonably be inferred from the data. The uses of epidemiology
In his celebrated monograph bearing this title, J.N. Morris (4> described seven uses of epidemiology. It continues to give us a framework for assessing the state of psychiatric epidemiology in relation to the biological and psychosocial conditions of the contemporary world. Morris's list can be reinterpreted for our use as follows in the next sections.
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