Physical properties of H II regions. Top panel: histogram showing how the distribution of the main measured properties of H II regions changes as a function of the considered sample. From the left to the right the following quantities are shown: S-calibration metallicities (Pilyugin & Grebel 2016), ionisation parameter (Diaz et al. 1991), circularised radius, Ha luminosity. 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 H II regions classified using the traditional classification criteria. Since there is no change in the number of H II regions in the full catalogue and the Hα sample, the two histograms overlap, and the blue line representing the full catalogue is not visible.