ICIDH
Estimates of the prevalence of disability derived from any study depend on the purpose of the study and the methods used. Since disability has no 'scientific' or commonly agreed upon definition, a major problem lies in the confusion over terminology. However, the International Classification of Impairments, Disabilities and Handicaps (ICIDH) represent a rationalisation of the terminology frequently used.
The ICIDH identifies impairment, disability and handicap as consequences of diseases and presents a classification for each. For example, disability is defined as:
Disability definition
"any restriction or lack (resulting from an impairment)
of ability to perform an activity in the manner or within the range
considered normal for a human being."