Left panel: the relationship between stellar mass and metallicity for the HRS sample of galaxies (blue circles), using metallicity estimates based on the PP04 O3N2 base calibration. The KE08 best fit (solid black line) and our linear and polynomial fits (dashed red and solid green lines, respectively) to the binned data (black squares) are presented. Error bars show the 1-σ scatter in each bin. Right panels: the residual metallicities as obtained from the linear and polynomial best fits.