Normalised histograms of the S/N for all APOGEE stars (shown in red) and for the stars with calibrated TEFF but without calibrated metallicity (shown in grey). For very low S/N values the stars without metallicity are clearly overrepresented compared to the overall distribution. At S/N ≳ 30 the distribution flattens, suggesting that the reason for failing to derive a calibrated metallicity is not low S/N.