ABSTRACT
The incidence rate of any given disease - i.e., the number of new cases which occur in a defined population unit during a specified time period - is of importance in the epidemiological search for causes, since this requires the frequency of illness-onset to be established in relation to the population's exposure to putative aetiological or risk factors. Because the occurrence of new cases, especially of chronic forms of disease, may be difficult to monitor directly, epidemiologists often have recourse to indirect measures, such as the numbers of new cases notified from medical practice, or of hospital first admissions. Data of this kind, however, will be subject to variability arising from diagnostic inconsistency, differences in the level of medical-care provision, and other extraneous influences.
