Physical properties of PNe. Top panel: histogram showing how the distribution of the main measured properties of PNe changes as a function of the considered sample. From the left to the right, the following quantities are shown: Hα luminosity, [O III]λ5007 absolute magnitude (adapted from Eq. (4) using distances from Table 1). The top row shows the absolute distribution for each quantity, while the bottom row shows the normalised distribution. Bottom panel: same histograms as shown in the top panel, but for the PNe classified using the traditional classification criteria.