Store-versus-store comparison is the standard way retailers evaluate coaching programmes and it cannot produce a result worth acting on, because the variance between two doors exceeds the effect being measured and pilot stores are almost never selected at random. This paper documents the alternative we run: cohorts inside a single store, where coached and capture-only associates share footfall, catchment, manager, inventory and promotional calendar, with a group swap at the midpoint to test whether the effect follows the coaching or the people. It specifies the preconditions that make the readout auditable — point-of-sale access at ticket level, cohort assignment fixed and written before counted measurement begins, and capture coverage published weekly so a thin week cannot be excluded after the fact. It also states the method’s limits: small cohorts, seasonal contamination, and what we do when coverage in a store degrades. Includes the readout template used at the day-90 decision review.