Intervention studies and observational studies are known as analytical studies. ‘Mapping health and disease’ emphasizes that these are generally preceded and followed by descriptive studies of how health and disease are distributed by place, in time, and in categories of people. Descriptive studies not only stimulate and guide analytical epidemiological studies but are a check that the results of the latter ‘make sense’. The description of health and disease in populations relies on ongoing and systematic data collection. In principle, all countries have a system of records of basic life events: births, deaths, and causes of deaths — however, the organization, coverage and quality of the data collected are very variable.