Effect of the m_r limit on the shape of the M − Z relation (left panel) and the color-stellar mass sampling (right panel). The gray symbols correspond to all targets in the volume-limited 0.07 <z< 0.1 sample, with a lower M_r limit of M_r = −18.4. The solid black line is the linear fit of the data set above the limit of representativity log M_∗= 9.4 given in Eq. (5). We mimic a 0.07 <z< 0.1 sample drawn from SDSS survey by gathering targets with m_r< 17.77 from 0.07 <z< 0.1 sample (red symbols). The red dashed line is the quadratic fit to the m_r< 17.7 data points. For reference, the fitted M − Z relation on 0.17 <z< 0.24 sample (the best fit shown in the lower left panel of Fig. 13 is plotted as a dotted blue line.