Correlation of SFE with various metrics of the dense gas component in clouds. In each case a power law is fitted (solid line), the fit errors are represented in light grey, and the (Gaussian) spread of the cloud of points with respect to the fitted line is represented in dark grey. Top left: SFE vs. column-density-based dense gas fraction. Top right: SFE vs. volume-density-based dense gas fraction. Bottom left: SFE vs. column-density-based density contrast. Bottom right: SFE vs. volume-density-based density contrast.