**3. Epidemiological concepts and strategies**

Epidemiology is concerned with the study of the distribution and determinants of disease frequency in human populations. The quantification of the occurrence of psychopathology in populations, can be regarded the central task of epidemiology. Well-known measures of frequency are *prevalence* (see 4.1) or *incidence*. Incidence quantifies the number of new cases with a disorder that develop in a population during a specified period of time. *Cumulative incidence* is the proportion of individuals who become disordered during a specified period of time (Verhulst, 1995). The distribution of disorders, involves comparisons between different populations or subpopulations. The examination of factors that are associated with variations in the distribution of psychopathology is essential for testing etiological hypotheses. Measures of association and risk are quantifications of the influence of certain factors on the occurrence of disorder. In follow-up studies measures of risk for developing a disorder using categorical data are *relative risk*, and *attributable risk*. In case-control studies the measure often used is the *odds ratio* which reflects the likelihood for developing a disorder in the group with a possible aetiological factor versus the group without this factor (Verhulst, 1995). For a more detailed discussion, see Verhulst and Koot (1992).

Epidemiological studies can be divided into prospective and retrospective studies, depending respectively on whether the measurement of exposure to a risk factor was done before or after the disorder occurred. A study, in which the presence or absence of a disorder and the presence or absence of associated factors are assessed at the same time, is called a cross-sectional study. If the aim of the cross-sectional study is limited to the determination of the prevalence, the study is called prevalence study.
