—As de Jong and Meerwaldt point out, Parkinson's disease disability is customarily rated on what are, strictly speaking, ordinal scales. To say that "average values cannot be calculated" is a misstatement. They are indeed calculated, not only in our article,1 but also in numerous other Parkinson's disease studies.2-9 The rationale for calculating mean values on these scores is based on the assumption that there exists an underlying continuum of disability. This is expressed in the UCLA score by a range beginning at O, for an absence of all 21 signs, symptoms, and impairments of daily living, with gradual increments representing increasing disability, to a maximum of 220 for the most severe manifestations. Other scores have different ranges.Hard measures, such as precise height or weight, have not been developed to assess most of the elements comprising Parkinson's disease disability. The majority of the rating systems use quantified expressions
Markham CH, Diamond SG. Modification of Parkinson's Disease by Long-term Levodopa Treatment-Reply. Arch Neurol. 1988;45(1):14. doi:10.1001/archneur.1988.00520250016004
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