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

Determination of a scale's factor structure requires a two-part process: (1 ) an initial examination of the factor structure using a sample of individu als with the condition of interest, and (2) repeated examinations of the fa ctor structure using the same analytic methods but applied to independent s amples 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 sect ion of the Unified Parkinson's Disease Rating Scale (UPDRS). We used a samp le of 294 consecutive patients with idiopathic Parkinson's disease (PD) ass essed while in the on-state and identified six clinically distinct factors. In the present study, we performed a confirmatory investigation of the fac tor structure and analysis of the internal consistency of the UPDRS in a ne w sample of 200 consecutive PD patients who were assessed while in the off- state. Factor analysis again revealed six factors with identical item loadi ngs as those obtained from examinations of patients in the on-state. Estima tes of internal consistency were comparable in the off- and on-state examin ations. These results indicate that the Motor Examination section of the UP DRS has a stable factor structure and high internal consistency across off- and on-state examinations.