I_N2H⁺/I_HCN ratios observed toward nearby galaxies (Aladro et al. 2015; Watanabe et al. 2014; Mauersberger & Henkel 1991; Nguyen et al. 1992; Eibensteiner et al. 2022; Nishimura et al. 2016b; Takano et al. 2019, and this work) and Galactic molecular clouds (Jones et al. 2012; Pety et al. 2017; Barnes et al. 2020; Yun et al. 2021). For galaxies with several measurements, we represent their mean values with associated uncertainties. The sizes of the circular markers are given in logarithmic scale. For visual reasons, the markers for the Galactic regions are scaled with 500 and the markers in the extragalactic regions with 300. The solid line represents the mean value among all samples, while the dashed lines indicate a 3σ dispersion from the mean.