Longitudinal modelling of symptom progression
Disease progression over the first five years was modelled, separately, in terms of severity and SoS. In the case of the severity, the longitudinal function was estimated from the item scores, while the difficulty and discrimination parameters were fixed to those determined as described above.
Informed by the data pattern shown in the upper panels of Figure 2, a linear function described in Equation 5 was used, whereSi (t ), was symptom, in terms of either Severity or SoS, for patient i at time t ;Si,0 was baseline; andSlopei was the progression rate. The IOVwas the inter-occasion (visit) variability to capture the fluctuation of clinical symptoms.
\(S_{i\left(t\right)}=S_{i,0}+\text{Slope}_{i}\bullet time+IOV\)Equation 5
Both severity and SoS longitudinal models were fitted to data, including or excluding the tremor items. (See Results section.)