Distribution of the ratios [OIII]5007/Hα (filled red rectangles and dots) and [OIII]5007/Hβ (filled white rectangles and gray dots). The thick black line represents the median value, and the box ranges from the first to the third quartile. The error bars represent the maximum and minimum value without outliers, which are plotted as dots (and defined as the values that lie beyond 1.5 times the interquartilic range). We plot in this figure results from our work and several samples from the literature (Yang et al. 2017; Cardamone et al. 2009; Amorín et al. 2015; Onodera et al. 2020; Hsyu et al. 2018; Guseva et al. 2017; Pérez-Montero et al. 2020; Matthee et al. 2021; Izotov et al. 2021), as well as the KISS survey (Wegner et al. 2003; Gronwall et al. 2004; Jangren et al. 2005; Salzer et al. 2005) and the Hα dots class (Kellar et al. 2012; Salzer et al. 2020). We selected the subsample with extremely low metallicity in Hsyu et al. (2018) (H18_XMP), as well as galaxies with EW(Hβ) > 100 Å in both Hsyu et al. (2018) XMPs (H18_XMP Hβ > 100 Å) and the KISS survey (KISS Hβ > 100 Å). Different samples are separated by vertical blue lines.