q_IR distribution of 3 GHz detections in the two highest M_⋆ bins (left and right panels), after correcting for the internal q_IR–redshift dependence. Only sources within complete z-bins were considered at each M_⋆. The total distribution (black histogram) was dissected among SF-dominated (blue) and AGN-dominated (red) radio sources, and fitted with two separate Gaussian functions (dot-dashed lines). While the SF population was fitted first, the AGN part was fitted in a second step from the total–SF residuals. The 1σ dispersion attributed to SF equals 0.20 and 0.23 dex at 10^10.5 < M_⋆/M_⊙ < 10¹¹ and 10¹¹ < M_⋆/M_⊙ < 10¹², respectively. Bottom panels: corresponding cumulative Gaussian fits, both normalised to unity. The vertical green dotted line marks the threshold that best separates between the SF and AGN populations (see Table 3), which rejects about 70% AGN and only 3−4% SFGs (open circles).