Ordinal scales and foundations of misinference.

Fundamental deficiencies in the information provided by an ordinal scale constrain the logical inferences that can be drawn; inferences about progress in treatment are particularly vulnerable. Ignoring or denying the limitations of scale information will have serious practical and economic consequences. Currently, there is a high demand for functional assessment scales within the rehabilitation community. It is hoped that such scales will satisfy the very real need for measures of function which reflect the impact of treatment on patient progress. Unfortunately, some commonly used evaluation instruments are not well suited to this task. The underlying rationale for clinical decision-making based on these scales is examined.