Surface density per lifetime, Σ_object∕τ_object, of Class II YSOs, Class 0/I protostars, and clumps in the three NGC 2264 subregions. Statistical lifetimes of 2, 0.2, and 0.3 Myr were assumed for Class IIs, Class 0/Is, and clumps, respectively. Error bars take into account statistical uncertainties, contamination level of 30% for Class IIs, an incompleteness level of 25% for the Class 0/I population in the central subregion, and a factor of two of uncertainty for the clumps. While the star formation activity of the northern subregion appears to be constant, it displays a burst in the central subregion and shows a tentative increase in the southern subregion.