Ratio of Eu-free stars over the total number of stars for bins of 0.5 dex in [Fe/H]. The model results are displayed with a black dashed line (model NS04), red solid line (model NS+MRD), and blue dotted line (model NS+SN). The red squares are the observational proxy for this ratio, so the ratio between stars in which Eu only presents an upper limit over the number of stars for which at least Ba has been measured. The error bars are only plotted to show the dimension of each bin in [Fe/H].