Effect of type 1 interlopers on the velocity estimates as a function of the number of galaxies for the gapper estimator. The velocity dispersion S_gap(N_gal) is computed first using the full galaxy sample (solid black line, equivalent to the red line in Fig. 2), and then it is compared to the estimates after clipping the galaxy sample at 3, 2.7, and 2.5 sigma in the velocity space.