Comparison between the EW_α from the models and from the observations. Colors represent K_dist, the distance to the Kauffmann et al. (2003) curve (upper panel), the OB stars proportion f(OB) (middle panel), and the ratio Q_{0 / 1} = Q(He⁰)/Q(H⁰) (lower panel). The solid blue line follows y = x. In the upper panel, the solid black curves enclose half of the models that are below the Kauffman curve (negative K_dist), while the dashed black curve encloses half of the models above the same curve (positive K_dist). In the middle panel, the solid black contour encloses half of the models ionized by OB stars (f(OB) > 0.5), while the dashed black contour encloses half of the models ionized by HOLMES (f(OB) < 0.5). In the lower panel, the solid black contour encloses half of the models ionized by OB stars (Q_{0 / 1} > 0.55), while the dashed black contour encloses half of the models ionized by HOLMES (Q_{0 / 1} < 0.55).