What are the strengths and weaknesses of currently used definitions and data collection methods on disability prevalence?

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For a cross-national insight into the scope and features of disability in persons at working age also two types of sources can be used:
(a) Surveys measuring (self) reported health and disability (e.g. in household panels, in living conditions surveys, in health surveys)
(b) Comparative data derived from social security administrations (on disability benefit/ pension recipients, benefit expenditures, etc.)

The two categories discerned do not give the same information on the same populations. Sources mentioned under (a) not only cover persons in employment; moreover, the prevalence of disabilities is measured as reported by the interviewed person, in the context of diagnosable conditions, ADL (activities of daily living) or participation. Moreover, cultural differences as to public awareness and attitudes towards persons with disabilities (e.g. stigma) may affect responses. The second source (b) only regards insured persons with disabilities that fulfilled the administrative and medical eligibility criteria. Further, a minimum loss of earning capacity and other client characteristics have been assessed by medical and vocational experts, and this loss is compensated. For this specific subpopulation comparatively more information is available (as to diagnoses, costs, employment, trends, rehabilitation measures, return to work efforts, etc.), but legal and institutional criteria that affect the inflow in the schemes differ considerably across countries.