Despite the significant improvement in AC1, Fig. \[fig-alfa-gass-4-par\] shows that no improvement in scatter or systematic offset is seen in the GASS dataset. The persistent systematic offset shown in Fig. \[fig-alfa-gass-4-par\] for the GASS data could be due to the two samples (AC1 and GASS) representing galaxies of different physical nature, so that even the matrix form of the estimator cannot extrapolate it in a suitable manner. This is demonstrated in Fig. \[fig-gr-mu\] where we plot $\mu_i$ vs. $g-r$ for the two samples; whilst there is some overlap, the GASS galaxies are preferentially located at the reddest $g-r$ colours and lowest values of $\mu_i$, that is poorly represented in AC1. It is possible to select a subset of the GASS data that are relatively well represented in the AC1 data, for which we might expect the calibration to perform better. In the lower panel of Fig. \[fig-gr-mu\] we again plot the comparison of estimated and observed  shown in Fig. \[fig-alfa-gass-4-par\], but now limiting the GASS data to the range $\mu_i>20$ and $g-r<0.6$, which is more populated by the AC1 sample. The estimated   does not remove all the offsets; however, this estimate is now closer to the observed one for this 