Complexity, case-mix and rehabilitation: the importance of a holistic model of illness.

Rehabilitation is not cheap. It depends upon personal interactions between rehabilitation staff and the patient and their families, and salaries for staff are always expensive. Rehabilitation depends upon learning, which takes time. The more complex the problems presented by patients the more resources are likely to be needed. This editorial reviews briefly the nature of complexity, emphasizing that it encompasses both the number of factors impinging upon the outcome of interest and the non-linear nature of many of the relationships between different factors and inputs. It describes briefly the holistic biopsychosocial model of illness that underlies much rehabilitation practice and a model of the rehabilitation process, and it then considers how complexity might be measured. It concludes that measures exist, such as the INTERMED, although they can probably be improved. But evidence derived using the INTERMED already both validates the biopsychosocial model of illness, and provides a sound basis for further developments.