Black points show the average stellar mass fraction for the low-mass COSMOS X-ray group sample computed by summing over the composite GSMF at each redshift. These values are compared with the average stellar mass fraction presented in Giodini et al. (2009) (dashed region). Gray points are the stellar mass fraction computed by Leauthaud et al. (2012), also on COSMOS X-ray detected groups but by using HOD analysis. The arrows show the upper and lower bounds of the systematic errors on the stellar mass estimates at low redshift as estimated by Leauthaud et al. (2012).