Distributions of the slope differences under different S/N cuts. The black solid histogram shows data with S/N of [S III], [O III], and Hβ greater than 3; the dashed green histogram shows data with S/N of the aforementioned lines greater than 10; the dotted red histogram shows data with S/N of the aforementioned lines greater than 15. The slope difference peaks toward smaller values with the increasing S/N limit. However, the difference is actually due to the dependence of the slope on the surface brightness of the lines.