Steps of our procedure applied to NGC 4833. Upper left panel: stars in NGC 4833 (Carretta et al. 2014a: filled blue squares) superimposed to field stars (Gratton et al. 2003: empty grey squares) in the [Ca/H]–[Mg/H] plane. Upper right panel: same stars (and symbols) after subtracting the linear fit in Fig. 1 from [Ca/H] to obtain linearised values [Ca/H]_corr. Lower left panel: generalised histograms of the [Ca/H]_corr values for field (filled grey area) and GC stars (empty blue area). Lower right panel: cumulative distribution of corrected Ca abundances for field and GC stars. The probability for the two-tail Kolmogorov-Smirnov test is also listed.