Distribution of errors of the stellar parameter diagnostics for the total sample. upper panels: propagated error on the measurement of the slope of Fe i abundances versus the excitation potential of the line (σ(slope_χ) left), and versus the line strength (σ(slope_W) left). lower panels: error on the mean Fe i and Fe ii abundances as measured by the dispersion around the mean divided by the square root of the number of lines of each species.