Top left: ratio between the estimated and actual masses of group-size dark matter haloes as a function of the fraction of group members with recessional velocity measurements, as obtained from the numerical simulations presented in this work. Top right: ratio between the estimated and actual masses of group-size dark matter haloes as a function of the number of member galaxies. Bottom left: ratio between the velocity dispersion as estimated with the gapper method and as obtained from the actual group members in the simulation. Bottom right: ratio between the deprojected virial radius and the mean 3D radius of actual groups members in the simulation, as a function of the fraction of members. The solid red lines show the results when using only members with absolute magnitudes brighter than M_r = −18, and the dotted blue lines for members brighter than M_r = −19. Errorbars enclose 68% of the measurements in each bin of fraction.