Defining the population, treatments, and outcomes of interest: reconciling the rules of biology with meaningfulness.

Population characteristics, treatment needs, therapeutic interventions, and outcomes are inextricably linked. To appreciate the treatment needs and outcomes of populations served by rehabilitation medicine, it is essential to understand how specific conditions impair mental and physical functioning, given the environments within which people choose to live. States of the mind and body combine with the characteristics of the man-made and natural world and the social infrastructure to yield disabilities and, thus, shape the demand for rehabilitation services. The 1997 draft of ICIDH-2: International Classification of Impairments, Activities, and Participation (ICIDH-2) is described as an approach to population definition and outcome assessment. A new and evolving model referred to as the spheres of human-environmental integration (HEI) is applied to expressing the nonlinear and overlapping relationships among the ICIDH-2 dimensions. HEI is defined as the individual's potential for meaningful physical and mental activity as determined by physical and mental capabilities in relationship to the man-made and natural worlds, social expectations, and available resources. HEI can be expanded by reducing disabilities through medical and rehabilitation interventions and by eliminating environmental barriers. This dual approach implies a need to integrate rehabilitation sciences with the principles of independent living, which view disablement as a function of the environment. The ICIDH-2 dimensions combined with HEI are used to define populations and to study the mechanisms and effects of alternative treatments through various techniques of case-mix measurement, disability staging, and utility assessment.