Star formation rate as a function of the molecular gas surface density for all sample galaxies. Upper panel: data with a constant CO-to-H₂ conversion factor. Lower panel: with normalized molecular gas surface densities (see text). The solid lines represent a linear regression 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. Triangles represent data points within the disk of NGC 4438 (RA offset >  − 20″), diamonds points in the western extraplanar region (RA offset ≤  − 20″; see Fig. 8).