Average values of the abundances [X_α/H] and [X_Fe/H] of the samples of SP, GP, and BDs, depicted as three-dotted-dashed lines in the top panels of Fig. 8, together with the values provided by zero-order fits to the weighted average of these element abundances, displayed as three-dotted-dashed lines in the bottom panels of Fig. 8. The error bars of the average values show Δ_σ = σ/, with σ equal to the standard deviation, and N the number of stars in each sample. The fit values have the errors of the coefficients of the zero-order functions.