The Functional Independence Measure (FIM) records the severity of disability of rehabilitation patients. The necessarily curvilinear relationship between the finite range of recorded FIM raw scores and the conceptually infinite range of additive disability measures is resolved through Rasch analysis. The analysis of admission and discharge FIM ratings of 14,799 patients shows that the 18 FIM items define two statistically and clinically different indicators. Thirteen items define disability in motor functions. Five items define disability in cognitive functions. Additive measures for each indicator have the same characteristics at admission and discharge, so that these measures can be used to assess change in patient status.