Relative number distribution of ATLASGAL groups in the first quadrant in panel a) and groups in the fourth quadrant in panel b) with the maximum velocity difference in a group for different input values of the velocity interval to search for associated sources in a group, Δv_i. We use 5 to 15 km s^-1 for that and a fixed maximum distance between individual sources in a group, Δd_i, of 0.3°.