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.