THE MULTIDIMENSIONALITY OF THE FIM MOTOR ITEMS PRECLUDES AN INTERVAL SCALING USING RASCH ANALYSIS

Rasch analysis scaling is said to produce an interval scale of Functio nal Independence Measure (FIM) motor function items. Rasch analysis re quires that the data to be analysed represent the influence of a singl e underlying unidimensional variable. A unidimensional interval scale of activities of daily living means that a person who can perform the most difficult item on a scale can also perform the easiest item. For a FIM motor function interval scale, the ability to climb stairs would imply necessarily an ability to eat normally. As this need not be the case, the FIM motor items do not constitute an adequate interval scal e. Eating and walking are different activities, and a unidimensional c onstruct linking them is unsatisfactory. A principal components analys is of the admission FIM motor function items of 515 consecutive patien ts admitted to an inpatient rehabilitation unit revealed that more tha n one significant factor was necessary to explain the variance in scor es. The counter-factual and statistical evidence argues that a unidime nsional construct does not underly the FIM motor function items, and t he use of Rasch analysis will not lead to a description of interval pr operties of the FIM motor function items.