The traditional incidence studies (“core studies”) establish a rate based on the general population residing within a defined geographical area. In register-based studies these are typically the number of new cases in a defined catchment area (often health district, nation etc) over a certain period of time.

In other studies, the sample is from a specially defined group – such as migrants, cohorts born within restricted intervals, or others such as the deaf population – we report these separately as they can help identify differences in rates. However we do not cover any incidence data from studies exploring risk factors such as prenatal famine, influenza and obstetric complications.