The epidemiology of anorexia nervosa and bulimia

EPIDEMIOLOGY is the study of disease in relation to populations. Unlike clinical studies which are carried out on patients, it requires study of both the sick and healthy in a defined population. A useful categorization of epidemiological research has been presented by COOPER and SHEPHERD (1973): (1) At the public health level epidemiological inquiry furnishes statistics concerning the extent of morbidity in a population, its locations and an assessment of the resources required by medical and other services to deal with it. At this level it also is concerned with prevention and therapeutic interventions in a population. (2) At the clinical level epidemiological research is mainly concerned with information about the course and outcome of disease. (3) At the aetiological level, epidemiological research aims at the detection of associations between the disease studied and other factors, thus suggesting causal hypotheses for further investigation. Rates are the hallmark of epidemiology and these provide the basis of comparisons between population groups. Rates are derived from two integers, a numerator and a denominator, with the study of each usually posing a number of problems. For the numerator, the problems concern the definition of what constitutes a “case”, while for the denominator the problem concerns the selection of a population or a probability sample of it, such that each member of the population has an equal chance of being included. Selection processes affecting subjects’ entry to a study need to be guarded against.