Left: scatter plot in the [Na/Fe] vs [O/Fe] plane for the high-resolution spectroscopy data set. Data are classified according to the MC resampling. Circles correspond to objects classified into group 1, and squares show objects classified into group 2. Colours were adopted to distinguish data according to the cluster membership probability. Purple symbols correspond to data for which the classification is uncertain (see text for details). Right: same as in the left panel, but in the [Mg/Fe] vs [Na/Fe] plane.