Observed differential core formation efficiency (CFE) as a function of background column density expressed in A_V units (blue histograms with error bars) in the Ophiuchus molecular cloud, in blue for the whole cloud, in orange for L1688, and in green for L1689. The CFE if obtained by dividing the total mass of prestellar cores in a given column density bin by the cloud mass in this column density bin. The vertical dashed line marks a fiducial threshold at .