Past and present issues in Rasch analysis: the functional independence measure (FIM™) revisited

To review the development of Rasch analysis by examining the history of its application to the Functional Independence Measure (FIMTM), and highlighting current issues in the approach.

All Rasch-based papers concerning the FIMTM were reviewed for their analytical strategy and results. Four analytical pathways were identified that accommodated the majority of these strategies. Data derived from secondary analysis of 340 in-patients undergoing rehabilitation following stroke, measured on the FIMTM Motor Scale, were fitted to the Rasch measurement model according to these 4 pathways, with 2 additional pathways to accommodate recent developments.

In the analytical pathway, where items are not re-scored, the fit to the Partial Credit parameterization was better than the Rating Scale version. Fit improved following re-scoring of disordered thresholds. When local dependency was accommodated by 4 testlets, the Partial Credit, re-scored testlet version achieved adequate summary fit with no misfit among items, and unidimensionality. All other pathways required item deletion.

The current study has shown that the FIMTM Motor Scale, as applied to a stroke rehabilitation sample, satisfies Rasch model expectations and the unidimensionality assumptions, having accommodated local dependency issues, and by using the partial credit parameterization with re-scored categories. Other analytical pathways gave less ideal solutions, and are consistent with the wide range of solutions found for the scale over the years. Consequently, the development of the Rasch approach in health outcomes can be traced in the history of analysis of the FIMTM, and that development continues to this day.