Best-fit parameters of the NFW mass model for the stacks in richness N (red triangles), luminosity L (orange disks), SIS velocity dispersion σ_v (blue squares), and arc radius R_A (purple stars). The large black rectangle shows the ⟨M₂₀₀⟩ ± 1σ and ⟨c₂₀₀⟩ ± 1σ obtained using the 80 groups (stack S0). The green-hatched area delimits the 1σ uncertainty around the mass-concentration relation derived with Lenstool, using a M₂₀₀(σ_v) scaling.