Efficiency of core formation measured as the fraction of mass that is inside the SPH sink particles as a function of the cloud gas density (extrapolated from Bonnell et al. 2011). Both the efficiency and the gas densities are measured in spherical shells centered on the densest region of the simulation and span size scales from ~ 0.04 to 10 pc. The points represent regions where the sink particles are all between 40 000 and 100 000 years old. The efficiencies increase with time such that older systems would have higher efficiencies, but the relation between SFE and ⟨ n_H2 ⟩ remains.