The cross shows typical errors of 0.3 in log₁₀M_c and 0.4 in log₁₀e. The diagonal dashed line shows the efficiency a cloud would have if associated with an HII region of luminosity L_min = 400 Jy kpc². The data are incomplete below this line. The vertical dashed line shows the molecular cloud survey completeness boundary at M_c = 4 x 10⁵ solar masses.