Trauma Ward Mental Health Open-Bay Versus Single-Room Admission and Post-Traumatic Stress Disorder Symptom Trajectories in South African Inpatient Units: A Target-Trial Emulation of Ward Milieu Exposure

South African trauma wards routinely admit patients to open-bay areas where sightlines, noise and the visible distress of others are unavoidable, yet the causal effect of this ward milieu on post-traumatic stress disorder (PTSD) symptom trajectories remains unestablished. No observational data were analysed; the study characterises the operating characteristics of the design under a declared variance structure with a between-cluster standard deviation of 6.0, a residual standard deviation of 4.5 and an intraclass correlation of 0.640. Power was estimated across 2000 replications per design cell for cluster counts of 8, 16, 24 and 32 and effect sizes of 3, 5 and 8 percentage points. The design required 54 clusters to detect a 5-percentage-point effect at 80% power. The BACI estimator is severely underpowered at the smaller cluster counts that characterise feasible South African ward studies, and that the minimum detectable effect falls steeply as clusters increase. The contribution is a bounded methodological finding: ward-milieu effects of the magnitude plausibly produced by open-bay exposure cannot be reliably detected in the cluster counts typically available, so the field must either accept larger multicentre designs or reframe the question around continuous symptom trajectories rather than binary caseness.