Left panels: upper and lower errors on [Fe/H] as a function of [Fe/H] for the 219 red clump stars. The upper values increase with the metallicity with mean values around 0.3  dex for the most metallic stars. The lower values show no trend with [Fe/H] with a median value of 0.17 dex for the whole sample. Right panels: upper and lower errors on [Mg/H] as a function of [Mg/H] for the 162 red clump stars selected from the total sample. At low [Mg/H], the abundance errors are dominated by the line measurement rather than stellar parameter uncertainties because the Mg triplet lines become very weak.