Longitudinal models for symptom severity and sum of scores
The longitudinal model described in Equation 5 was successfully fitted to both severity data and SoS data, with or without tremor items. The symptom progression rates for both severity and SoS were in turn found to be functions of the baseline: patients with worse symptom at baseline appeared to have slower progression. When all items were included in the modelling, the progression rate of severity, for the typical patient with a baseline of zero point, was 0.227 points per year. The progression rate of SoS, for the typical patient with a baseline of 19.6 points, was 2.99 points per year. When the tremor items were excluded, the progression rates for severity and SoS were 0.243 and 2.24 points per year, respectively. All model parameters are listed in Table 3.