Relationship between the ratio of the total CO flux as determined from the extrapolation of the central beam observation to the total value determined from the maps for galaxies with integrated data from Kuno et al. (2007) as a function of the filling factor ff, defined as the fraction of the galaxy covered by the central beam (lower panel). Different symbols are used for the galaxies observed in the FCRAO survey of Young et al. (1995; cyan), Stark et al. (1986; magenta) and other references (black). Filled dots are for galaxies with D₂₅(B)/θ ≤ 5, empty symbols for those objects with D₂₅(B)/θ > 5. The solid line shows the one ratio, the long dashed lines the typical 1σ uncertainty (44%) for galaxies where the beam size covers more than 10% of the total optical surface of the galaxy, the dotted line 1σ uncertainty (123%) for galaxies where the beam size covers less than or equal to 10% of their surface. The upper panel shows the distribution in the area covered by the central beam for all HRS galaxies with available CO data (black histogram). The blue histogram gives the distribution of all galaxies observed by our team, including previous data (Boselli et al. 1995, 2002; Sauty et al. 2003), while the red one only those observed in this work.