Venn diagram combining the samples of AGN selected by means of different diagnostics: optical variability (V), X-ray properties (X, from the X/O diagram), Lacy region (L), and Donley region (D). The circle sizes, as well as the overlap sizes, are quantitatively correct. The sample of optically variable sources includes the 34 sources that are not confirmed by any other diagnostics, while it does not include the 9 sources that are confirmed as stars (see main text). It is apparent that the variability-selected sample largely overlaps the X-ray selected and the Lacy-selected AGN, while the overlap with the Donley-selected sample is only partial.