Molecular fraction Σ_H2/Σ_HI as a function of the total gas surface density Σ_g = Σ_H2 + Σ_HI for all sample galaxies except NGC 4438. Upper panel: data with a constant CO-to-H₂ conversion factor. Lower panel: with SFR/L(CO)-normalized molecular gas surface densities (see text). Symbols of different colors represent different galaxies. Black symbols represent the two anemic galaxies NGC 4548 (triangles) and NGC 4579 (diamonds). The solid lines represent a linear regression to the data of all galaxies except NGC 4548 and NGC 4579 using a Bayesian approach to linear regression with errors in both variables (Kelly 2007). To guide the eye, the dotted lines represent offsets of a factor of two.