The Barthel Index: an ordinal score or interval level measure?

Abstract

The Barthel Index is one of the most widely used activities of daily living (ADL) measures in stroke rehabilitation and there has been some debate recently about whether or not the Index is an ordinal score or an interval level measure. An audit of 192 consecutive patients undergoing inpatient rehabilitation following stroke has provided the opportunity to examine this question with recently developed mathematical techniques based on the work of George Rasch. Rasch models define the criteria which data must follow to produce an interval level measure. It thus becomes possible to test the data derived from the audit against the Rasch model. Calibration of the 10 items in the Index shows considerable differences in the degree of difficulty (weight), and these differences are not compensated for by the current scoring. Thus adding together the items produces a scale whose intervals vary considerably, particularly between intervals at the lower or upper ends of the scale, and those at the centre. This can give rise to considerable differences between the change score based on the Rasch transformation (taking into account item difficulty) and the change score based on raw scores. These findings confirm the ordinal nature of the Barthel Index. Further questions are raised about the unidimensionality of the Index, and the context in which it should be used.