Longitudinal investigation of the factor structure of the Parkinson's disease activities of daily living, interference and dependence instrument

The Parkinson's Disease Activities of Daily Living, Interference, and Dependence Instrument© (PD-AID) is a patient-reported outcome (PRO) instrument, recently developed to assess the clinical benefit of Parkinson's Disease (PD) treatment. The PD-AID consists of morning and evening assessments, administered daily. To benefit from the full set of the repeated observations over time, analytic approaches that account for both within- and between-individual variability are required. The current study aimed to employ the advantages of exploratory Multilevel Factor Analysis (MFA) on data collected from 93 participants with moderate to advanced PD, currently using and responding to Levodopa (L-Dopa), who completed the PD-AID twice daily as part of a prospective, non-intervention, observational study for ~28 days. Average daily completion rates were comparable for the Morning and the Evening PD-AID (78% and 74%, respectively). The intraclass correlation coefficients for the Morning and Evening PD-AID items were in the range of 0.70–0.90, with an average of 0.81 for the Morning PD-AID items and 0.83 for the Evening PD-AID items, suggesting that most variability (81%–83%) in responses was due to between-individual variability. For the Morning PD-AID, one factor (including nine out of 10 Morning PD-AID items) emerged at the between-individual level and four factors (core physical actions, basic self-care activities, feeding, and interference & dependence) at the within-individual level. For the Evening PD-AID, there were four between-individual factors (basic activities of daily living ADLs, life interference, impact on planning, and emotional consequences) and five within-individual factors (basic ADLs, toileting, life interference, medication planning, and emotional impact). The factors had high reliability.