The International Classification of Functioning Disability and Health: its development process and content validity.

The World Health Organization's International Classification of Functioning, Disability and Health (ICF) has provided a new foundation for our understanding of health, functioning, and disability. However, different challenges have to be addressed during its implementation process. The objective of this paper is to address two of these challenges, namely, the study of the content validity of the ICF and its relationship to other health-related concepts such as well-being, quality of life (QoL) and health-related quality of life (HRQoL). Ongoing validation studies confirm that the ICF is an exhaustive classification, i.e. it covers most of the health and health-related domains that make up the human experience of functioning and disability, and the most environmental factors that influence that experience of functioning and disability. The ICF also contributes to the understanding of health on a continuum ranging from a body-centred view (''the bodily experience of health''), to a more comprehensive perspective (''the entire health experience''), and finally, to an overarching view (''the human experience'') which sees health as part of the human condition. The ICF allows the operationalization of health as part of the human experience on this continuum as health from a narrow perspective to the broad perspective of functioning. The ICF with its categories can also serve as starting point for the operationalization of objective well-being. Since HRQoL can be defined as an individual's perceptions of health and health-related domains of well-being, the ICF categories encompassed in the ICF concept of functioning can also serve as the basis for the operationalization of HRQoL.