Physical properties of shock-ionised regions. Top panel: histogram showing how the distribution of the main measured properties of SNRs changes as a function of the considered sample. The following quantities are shown from the left to the right: Hα luminosity, [N II]λ6584 velocity dispersion and Hα velocity dispersion. 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 SNRs classified using the traditional classification criteria.