Objectives: To investigate the dimensionality of functional and community outcomes following serious TBI. To identify items that fit, misfit or are redundant, as well as to assess person misfit.
Methods: Rating-scale (Rasch) analysis was applied to 1-year follow-up data from 231 cases in the US National TBI Model Systems database. Items selected for analysis included all items indicative of global outcomes, disability, activity or participation.
Results: A powerful singular measurement dimension was identified. Item reliability was very high (0.98), as was person reliability (0.97). The dimension fit over 90% of cases; that is approximately 10% of cases displayed anomalous patterns of functioning that indicated that their functioning was not measurable in terms of the general dimension identified. There was tension within the dimension between ratings of dependency (FIM) and cognitive functioning in everyday life (NFI). Most-but not all-neuropsychological test scores misfit the outcome dimension.
Conclusions: Actual dimensionality was distinct from the named scales employed. A unidimensional measure model fit the data much better than expected. This outcome dimension might be called 'general community functioning'. In the future, it should be possible to develop more valid and parsimonious measures of community outcomes following TBI.