Factor analysis of the motor section of the unified Parkinson's disease rating scale during the off-state

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Abstract
Determination of a scale's factor structure requires a two-part process: (1) an initial examination of the factor structure using a sample of individuals with the condition of interest, and (2) repeated examinations of the factor structure using the same analytic methods but applied to independent samples of individuals with the condition of interest who contribute unique variability to the scale measurement. In a previous study, we performed an initial investigation of the factor structure of the Motor Examination section of the Unified Parkinson's Disease Rating Scale (UPDRS). We used a sample of 294 consecutive patients with idiopathic Parkinson's disease (PD) assessed while in the on-state and identified six clinically distinct factors. In the present study, we performed a confirmatory investigation of the factor structure and analysis of the internal consistency of the UPDRS in a new sample of 200 consecutive PD patients who were assessed while in the off-state. Factor analysis again revealed six factors with identical item loadings as those obtained from examinations of patients in the on-state. Estimates of internal consistency were comparable in the off- and on-state examinations. These results indicate that the Motor Examination section of the UPDRS has a stable factor structure and high internal consistency across off- and on-state examinations.