Combined effect of type 1 and 2 interlopers on S′_X(N_gal)/S_std(< R₂₀₀) as a function of the number of galaxies. The velocity dispersion, S′_X(N_gal), is first computed using the full galaxy sample (equivalent to the three colour lines shown in the left panel of Fig. 3). We also evaluated the response of biweight (blue), standard deviation (green), and gapper (red lines) using samples contaminated by a certain fraction of type 2 interlopers, as described in the text. By construction, our simulations also include the type 1 interlopers because the velocity dispersion is estimated in the cylinder.