Top: 21 μm-to-1.2 mm flux ratio versus the normalized angular separation between millimeter and infrared sources (see Sect. 6.1 for details). The flux at 21 μm flux is obtained from the MSX catalog and the flux at 1.2 mm is obtained from Beltrán et al. (2006). Bottom: standard deviation in the 21 μm-to-1.2 mm flux ratio versus the normalized angular separation (black dots). The dashed horizontal blue line corresponds to the standard deviation (≃0.57) for data with Δ_n > 0.7. The gray histogram indicates the probability that the distribution of data within one bin is the same as the global distribution with Δ_n > 1, using the Kolmogorov-Smirnov statistical test. The red vertical line indicates the normalized angular separation that distinguishes between millimeter and infrared associated and non-associated sources (see Sect. 6.1).