ABSTRACT
One of the most impressive facts concerning the study of old age is the rapid increase of problems in this field, certainly in old-age psychiatry. The prevalence of a disease depends, moreover, on its morbidity and especially on its duration and rate of mortality-the latter insofar as it differs from the average rate for the age group concerned. The prevalence figures have, of course, a still lower correlation with the disease expectancy than the incidence figures have. Epidemiological studies may aim at determining disease expectancies, incidence figures, or prevalence figures. The expectancy figures are of the greatest biological revelance; incidence figures may be of biological significance, but all of these figures, especially the prevalence figures, are greatly influenced by social factors.
