Two-dimensional star formation efficiency, ϵ_2D (Eq. (23)) against its three-dimensional counterpart, ϵ_3D (Eq. (22)). The (black) dotted and solid lines correspond to ϵ_2D = ϵ_3D and ϵ_2D = ϵ_3D − 0.20. ϵ_2D, the observed efficiency, is lower than ϵ_3D, the actual efficiency, because surface densities measured around the clump centre intercept clump outskirts where the gas volume density and ϵ_3D are lower.