Low- and high-flux sample distribution in different optical diagnostic (BPT) diagrams. Left: [NII]-based diagnostic diagrams of the parent (grey) and Effelsberg samples: a low-flux sample represented by black points and a high-flux sample denoted by green crosses. Demarcation lines were derived by Kewley et al. (2001) to set an upper limit for the star-forming galaxies and by Kauffmann et al. (2003) to distinguish purely star-forming galaxies. The dividing line between Seyferts and LINERs was derived by Schawinski et al. (2007). The new Effelsberg sample (black points), extended towards lower radio fluxes, covers the whole diagnostic diagram. Middle: same samples as in the left panel in [SII]-based diagnostic diagram. Right: high- and low-flux samples in [OI]-based diagram.