Velocity dispersion σ of a bright and a faint sample as a function of the dividing magnitude m_V. The two upper panels show objects in the full radial range, the lower two only show objects with R < 5′ (cf. Fig. 13). The number of objects contained in the bright/faint sample is indicated next to each data point. The comparison is done for both the full sample (filled symbols) and the outlier-cleaned sample (open symbols).