Top: mean fraction of protostars in a given class as a function of time. The thick semi-transparent lines in the background show the results for each cloud, whereas the black/colored lines show the results for the total population. Observed fractions are shown as dots. The fraction of Class III sources is obtained as N_III = N_tot − N_II − N_flat − N_I − N₀. Middle: absolute number of stars in each class as a function of time for all clouds together, for the star formation rate provided in Table 3. Bottom: same as middle panel, but with a cut-off time of 2.5 Myr after which the star-formation rate is set to zero, as indicated by the vertical dashed line. When the rate is zero, the total number of stars naturally remains constant.