Epidemiological research aims to establish the causes, mechanisms, and natural history of illness. It has traditionally focused on specific diseases, using data obtained from populations to investigate the factors related to the disease of interest. The relevant data will usually include some of: the characteristics of the patient; the manifestation of the disease; the patient’s history and social status; the patient’s past and present environment; and the characteristics of other people without the disease. Analysis aims to establish the nature and strength of the interrelationships between these factors and the occurrence of disease. In this way epidemiology has answered questions such as “Why does this disease occur? What causes it? What is likely to happen? How can we prevent disease or improve recovery”

Often epidemiological research will incidentally also answer the question, “How common is this disease?” Consequently a second, much more recent, focus has been the use of epidemiological data on the frequency (incidence or prevalence) of diseases to plan the delivery of health services. This secondary analysis has tried to answer the questions: “How many people will develop or have the disease? What services may be needed by patients with this disease?” However, most of the data used come from studies which were designed to disentangle the causes or natural history of disease. Very few studies have set out to determine the need of the community for specific health services, not least because it is difficult to determine a person’s need for an intervention (defined as their ability to benefit from the intervention) from the information available.

More broadly, epidemiological research also “strives to develop a theoretical framework for the understanding of health experience”.1 Disability is, for many people with neurological disease, an integral if not overwhelming part of their health experience. Therefore this review starts with …