Developing a rehabilitation intervention difficulty index: A mixed-methods study using NASA-TLX and Borg RPE in a tertiary clinical setting

Background: Rehabilitation difficulty varies substantially across clinical areas and intervention types, yet few brief, standardised measures exist to quantify difficulty from the therapist’s perspective. Understanding rehabilitation difficulty is important for workforce planning, resource allocation, and identifying contexts where therapist support is most needed. Objective: To develop and preliminarily evaluate the Rehabilitation Intervention Difficulty Index (RIDI), a mixed-methods measure combining cognitive/affective and physiological dimensions of rehabilitation difficulty, and to identify contextual factors associated with high difficulty. Methods: Quantitative component: 441 rehabilitation sessions from 28 therapists across 12 clinical areas were assessed using the NASA Task Load Index (NASA-TLX; four items: mental demand, temporal demand, effort, frustration) and Borg Rating of Perceived Exertion (RPE). The RIDI was constructed as the mean of the NASA composite score and normalised Borg RPE (0–10 scale). Psychometric properties were evaluated using Cronbach’s alpha and exploratory factor analysis. Descriptive statistics, one-way ANOVA, Pearson correlations, and intervention-level summaries were performed. Results: Quantitative findings: RIDI showed good internal consistency for the NASA items (α = 0.80, 95% CI [0.77, 0.83]) and acceptable consistency for the five-item set (α = 0.75, 95% CI [0.72, 0.78]), with a unidimensional factor structure explaining 52% of the variance. RIDI scores differed significantly across clinical areas (F = 9.18, p