To guide discussion related to disability, we turn to the International Classification of Functioning, Disability, and Health (ICF) to provide a framework for the concept of disability (WHO 2001). The ICF model draws on the medical and social models of disability, describing disability as the result of an interaction between health conditions and environmental factors. According to the ICF model, disability can occur at any of the three levels of functioning: body or body part (impairment), the whole person (activity limitation), and in a societal context (participation restrictions). The presence of disability is further dependent on contextual influences, including environmental and personal factors. Environmental factors such as physical structures, technological aids, laws and policies, and social attitudes may alleviate or contribute to disability at each level. Similarly, personal characteristics such as age, education, coping styles, and occupation may lead one person with a health condition to be regarded as having a disability while another person with the same condition is not regarded as having one. The levels of disability defined in the ICF model do not necessarily build upon one another. That is, it is possible to have an impairmentbut not a participation restriction, and vice versa. For example, a person with HIV with no impairments or activity limitations may be denied employment based on his health condition, creating a participation restriction (Weathers 2009).
The concepts defined in the ICF model are similar to those defined in a model of disability developed by Nagi(1965). Nagi posits that disability is the manifestation of a health limitation in a social context. Nagiexplains that not all impairments or functional limitations precipitate disability, and similar patterns of disability may result from different type of impairments and limitations in functions. Both the ICF and Nagi models acknowledge the importance of the environment in determining disability, and recognize that disability can occur on several levels. The difference between the two models is in terminology and the boundaries at which various levels of disability are defined (Mathiowetz and Wunderlich 2000).
Translating the concepts developed by the ICF and Nagi models into a standard set of survey questions to identify disability is difficult. In general, capturing the complexity of disability in a concise set of questions to be included on a survey is challenging. Further, conceptual models of disability differ from the many programmatic definitions of disability; the Interagency Committee on Disability Research (ICDR) documented 67 different administrative definitions of disability (CESSI 2003). Due in large part to the lack of a uniform definition of disability, definitions of disability on surveys vary as well, as is documented in the following section.