Observed differential core formation efficiency (CFE) as a function of background column density expressed in A_V units (blue histogram with error bars), obtained by dividing the mass in the form of prestellar cores in a given column density bin by the cloud mass observed in the same column density bin. The right axis gives the corresponding star formation rate per unit gas mass (in units of yr^-1), estimated assuming a local star formation efficiency ϵ_core = 0.4 at the core level and a prestellar core lifetime t_pre = 1.2 Myr (see text). For comparison, the black curve shows the prediction of the multi-freefall version of the turbulence regulated model of the star formation rate initially proposed by Krumholz & McKee (2005) (see Hennebelle & Chabrier 2011). The vertical dashed line marks the same fiducial threshold at 7 as in Fig. 11. The horizontal dotted line marks the rough asymptotic value of the CFE ~ 15% at A_V> 15, corresponding to a star formation rate per unit gas mass ~ 5 × 10^-8 yr^-1. (Color figure is available in the online version.)