Which measure of disease occurrence?

A key aspect of epidemiology is about understanding and exploring variations in disease. In order to do so it is important that the same definitions of disease occurrence are used, no matter where a study is being conducted, and that the same approach to measurement is taken. The epidemiologist needs to be aware of which measure to use in specific circumstances. Measuring disease occurrence is the basic activity of epidemiology, and the following section provides the background to choosing the most appropriate measure(s) of disease occurrence for the aims of the study. The term ‘disease’ can also be taken in this context to describe any personal attribute. Thus, the concepts described in this chapter apply equally well to assessing the occurrence of a symptom, the development of a particular disability, or the frequency of a medical intervention.