[Fe/H]_ANN − [Fe/H]_Cat versus their catalog values, [Fe/H]_Cat for all calibrated stars (top panel). The middle panel shows the result for part 1 obtained using weights from the ANN trained for part 2. In the bottom panel the weights from part 2 are applied to part 1. The rms error for the full sample (top panel) is 0.15 dex, for part 1 (middle panel) it is 0.31 dex, and for part 2 (bottom panel) it is 0.22 dex.