Relative number distribution of ATLASGAL groups with the size of a group for different input values of the maximum distance between individual sources within a group, Δd_i. This is varied between 0.05° and 0.5° with a fixed interval for velocities of neighbouring sources in a group, Δv_i, of 10 km s^-1 for the sample in the first quadrant in panel a) and the groups in the fourth quadrant in panel b).