Percentage of free-free emission at 1.3 mm (P_ff) as a function of the spectral index of free-free emission (α_ff). A group of cores with a higher spectral index, α_ff > 0.2, can be distinguished, but P_ff and α_ff are not anti-correlated. The core sample represented here is made up of 23 cores with H30α and X band detections. Cores with H/UC HII regions and HMCs are shown in red, and cores with H/UC HII regions only are shown in blue.