Day-to-day variability in naturalistic key-hold duration is associated with self-reported Parkinson’s disease impact

Background Naturalistic keyboard use enables repeated, low-burden assessment of fine-motor behavior. Whether day-to-day variability in key-hold duration provides information beyond average key-hold duration in Parkinson’s disease (PD) remains unclear. We tested whether within-person variability in daily key-hold duration was associated with a one-time self-reported PD Impact category. Methods We reconstructed the public Tappy dataset from raw keystroke files. The primary phenotype was the within-person coefficient of variation (CV) of daily mean key-hold duration. Ordinal-trend and categorical Impact parameterizations were treated as parallel primary analyses. Secondary and exploratory analyses evaluated alternative variability measures, timing-filter thresholds, observation density, robust daily summaries, approximate disease duration, and unequal precision of daily means. Participant-day location-scale models used both a conventional finite-keystroke sampling-variance approximation and a session/block-aware alternative; participant-cluster bootstrap inference used 2,000 replicates. Results Quality control retained 8,999,669 keystrokes and 5,490 valid participant-days. The complete-case analysis included 73 participants with PD (33 Mild, 30 Medium, and 10 Severe). Each one-category increase in self-reported PD Impact was associated with a 32.4% higher day-to-day CV (ratio, 1.32; 95% CI, 1.13–1.55; P < 0.001). In the categorical parameterization, ratios were 1.38 for Medium versus Mild and 1.70 for Severe versus Mild (overall robust Wald P = 0.001). Estimates were essentially unchanged with 1,000- or 1,500-ms timing limits and a hold-duration-only filter. Daily-median CV remained associated with Impact (ratio, 1.30; 95% CI, 1.08–1.56; P = 0.005). Among the same 52 participants eligible at 500 keystrokes/day, the ratio was 1.25 using the 100-keystroke definition and 1.20 using the 500-keystroke definition. In 72 participants contributing 3,193 participant-days, the session/block-aware location-scale model estimated a 1.62-fold higher latent residual SD per Impact category (2,000-replicate participant-cluster bootstrap 95% CI, 1.20–1.98). Conclusion Greater self-reported PD Impact was associated with higher within-person variability in naturalistic key-hold duration. This cross-sectional association identifies day-to-day variability as a candidate feature for passive fine-motor assessment. Prospective validation against repeated clinical outcomes and independent naturalistic cohorts is needed.