Cumulative fraction of stars as a function of the N₂H⁺ intensity (I₀). The distribution of Class 0 (orange), Class 0/I (i.e., all protostars; red), and Classes Flat (green), II (blue), and III (black) are independently displayed. The two shaded areas (with 1σ, 2σ, and 3σ contours) correspond to the cumulative fraction of objects resulting from random distribution of objects (grey) and a distribution where the probability of finding an object is proportional to the second power of the N₂H⁺ intensity (light blue) (see text). The two vertical lines define the N₂H⁺ intensity of the first (dotted line) and second (dashed line) contours in Fig. 5, respectively.