Relationship between molecular gas and atomic gas deficiency for spiral galaxies when the molecular gas mass is determined assuming a constant X_CO factor (panel a)) or the luminosity-dependent X_CO factor given in Boselli et al. (2002) (panel b)). Symbols are as in Fig. 1. The horizontal dotted lines show the one-sigma dispersion of H₂-deficiency parameter as determined from the calibrating relations given in Fig. 2.