The myth and reality of disability prevalence: measuring disability for research and service.

The concept of disability has medical, functional, and social components. We review the frameworks for the definition of disability and endorse a multidimensional approach that is parsimonious and has utility for epidemiology. We need to be able to count people with disabilities to quantify service and support needs, to study the life course of people with specific disabilities, and to accurately target prevention strategies. In addition, it is important to have some precise measures of disability so comparisons can be made between impairments, to identify disparities and differences, and to measure changes over time. We need to ensure that there is no confusion about cause and effect (attributing associated conditions as outcomes of disability when they are in fact the cause of the functional limitation) and that comparisons to people without disabilities are fair. If people who experience disability due to a condition such as obesity are included in the case definition of disability, then we cannot say people with disability are more likely to be obese. The credibility of disability epidemiology and the disability field is at stake.