Comparison of different star types and correlation methods. Left panel: abundances of the four standard stars: HD 122563 ([Eu/Fe] ≃ −0.64), HD 11544 ([Eu/Fe] ≃ 0.68), CS22892−052 ([Eu/Fe] ≃ 1.53) and CS31082−001 ([Eu/Fe] ≃ 1.62), from the JINA database and normalized to the absolute abundance of Fe from CS22892−052. log ϵ of element X is defined via its ratio with respect to the hydrogen abundance (log ϵ(X) = log(N_X/N_H) + 12). Right panel: Pearson and Spearman correlation coefficients of Fe and Eu in stars with [Fe/H] < −2.5 as a function of the upper limit for the [Eu/Fe] interval utilized. At [Eu/Fe] ≃ 0.3 the Pearson and Spearman coefficients start diverging from each other. The position of the four typical stars HD 122563, HD 115444, CS22892−052 and CS31082−001 are indicated.