Local surface density of YSOs, Σ _⋆ , in dependence on the local surface density of the unprocessed/observed gas, Σ_g. Top panel: models for a molecular clump of mass M₀ = 10⁴   M_⊙, radius R = 6   pc, density index p₀ = 1.7, a star formation efficiency per free-fall time ϵ_ff = 0.1 and the times t quoted in the key. The dotted (black) line depicts the average star-formation law inferred by Gutermuth et al. (2011). The orange and brown polygons illustrate the associated scatter observed for the MonR2 and Ophiuchus molecular clouds, where it is the smallest. Middle panel: models for a time t = 2 Myr and three distinct density indices of the molecular clump, p₀ = 1.5, 1.7, and 1.9. The normalization of the model hardly depends on p₀. Bottom panel: same as top panel but completed with the time evolution of the gas and YSO surface densities at four projected distances s from the clump centre (s-labelled dotted black lines).