3D plot of our approach for selecting candidate CGs. The members of a generic CG are displayed with magenta spheres and the surrounding Cepheids with gray spheres. The dark gray inner area shows the sphere including the members of the CG (the radius R_CG). The light gray outer area shows the spherical layer with a volume three times larger than the volume of the selected CG. Candidate CGs have a Cepheid density that is 3.5 larger than the average stellar density of its neighborhood.