Using a theory-based approach to evaluate a physical activity promotion intervention for youth with intellectual disabilities.

Youth with intellectual disabilities are less physically active than their typically developing peers. Although interventions to promote physical activity (PA) exist, their effectiveness remains limited and poorly understood. This study introduces a theory-based single-case evaluation approach, using a co-created dyadic PA intervention as the empirical setting to examine causal mechanisms and contextual conditions underlying intervention (in)effectiveness at the individual level. Seven young adults (Mage = 18.9 ± 1.7 years) with mild to moderate intellectual disabilities participated in a single-case (A1-B-A2) evaluation with an average 8-week intervention phase. PA was measured using accelerometers and analysed through visual analyses and overlap indices. An initial program theory outlining hypothesised causal steps and contextual conditions was developed before analysis. Biweekly interviews with participants and post-intervention interviews with buddies and caregivers (n = 78) were analysed to refine the program theory. Quantitative analyses indicated increases in total PA in two of seven cases, while three lacked sufficient accelerometer data. The refined program theory outlined three overarching principles (goal interaction, sense of belonging, caregiver support) and six causal steps from 'buddy compatibility' to 'activity exploration' and 'integration'. Across this causal chain, 25 support factors, 29 derailers, and 26 safeguards were identified. The analysis revealed distinct case-level constellations explaining heterogeneous outcomes. Beyond mixed effects on PA, this study shows how a theory-based single-case evaluation can generate mechanistic and context-sensitive insights in heterogeneous populations. Future research should build on these insights to further optimise the intervention and examine feasibility in larger samples.