Distribution of clusters in the training set as a function of Σ (units counts arcsec⁻²), and the average surface brightness in the 90″ radius around their centre. The x-axis is rescaled with c = 2 × 10⁻⁴ arcsec⁻². Top panel: histogram of all simulated clusters (blue) and histogram of the subset of those found as extended by eSASS (orange). Bottom panel: dots indicate the ratio of the histograms (empirical selection rate). Error bars are the 68% confidence range estimated according to Appendix C. The vertical dashed line indicates the transition $\widehat{\eta }=1.94$ in the logistic model described by Eqs. (3) and (4) – corresponding to N ≃ 30 counts.