Fraction of galaxies that are a) classified; b) classified as SFN; c) classified as TO; d) classified as AGN, with respect to the noise level using two different classification methods. The first method is the one presented in this paper that takes into account the information provided by upper limits (solid line, square markers) and the second method is the classical method that takes into account only galaxies with detections of all lines involved in the [N ii] diagnostic diagram (dashed line, round markers).