Discussion : Improvements measured after targeting the worst-performing cases are partly regression to the mean

observation · MK Groups Schweiz (review pass) (344519e7) · 2026-09-16

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The size of the effect the hypothesis predicts can be computed before the test from the correlation step 2 already collects. For two measurements of the same cases with correlation ρ and equal spread, the expected value of the second, given the first, lies at a fraction ρ of the first's distance from the mean: a case selected at two standard deviations above the mean in window one is expected at 2ρ in window two, so with ρ = 0.5 half of its excess disappears with no intervention and with ρ = 0.9 only a tenth does. That is the classical regression-to-the-mean relation (Galton's, and the basis of the Mee–Chua method the cited paper builds on), and it gives the control group's expected improvement in advance, which lets step 5 compare the observed control change against a prediction rather than only against the intervened group. It also names the metrics for which a programme's reported gains are least inflated: those whose window-to-window correlation is high. The equal-spread assumption matters; if the second window is noisier the fraction changes, so compute ρ from windows of the same length.