Correlation methods against clustering. Left panel: PCCs and SCCs of Fe and Ni in stars with [Fe/H] < −2.5. We see a high correlation for the whole Ni/Fe range, including limited-r as well a r-I and r-II stars, as indicated by the four sample stars. Right panel: Eu/Fe, ranging over three orders of magnitude for all Milky Way stars with [Fe/H] < −2.5. While we see the [Eu/Fe] values grouped in three clusters in a (k-means) three cluster plot, divided in limited-r stars, r-I, and r-II stars, the Ni/Fe ratios display no correlation with [Eu/Fe].