Top panel: offset between the SFR derived from the Class I population and the true SFR as a function of the ratio between the numbers of Class I and Class III PMS stars. The figure contains data from 250 clusters, each containing N_* = 500, and for each cluster, it includes measurements at 100 epochs. The SFH is flat and the IMF resembles that of the Milky Way field. Bottom panel: similar to the top panel but showing the offset between the SFR derived from the Class II population and the true SFR as a function of the ratio of the numbers of Class II and Class III PMS stars.