Left panel: n_e derived from 6 cm radio continuum emission versus n_e from mm-RRLs. Equality is indicated by the black dashed line. The dotted lines show the thresholds of electron density for UC Hii. The colors represent H ii region physical diameters in units of pc (see color bar) and the purple circles mark regions with diameter over 0.5 pc. The red line represents the best-fit line determined by an xy-bisector fit to the data. Right panel: n_e as a function of H ii region diameter. The red and green symbols show the distribution of the same sample of mm-RRL detections but where the election densities are derived from the mm-RRL emission and 6 cm radio continuum emission, respectively. The black symbols indicate data from previous radio continuum surveys and the blue circles are radio sources toward which no mm-RRL emission is detected and where the electron density has been estimated from the of 6 cm radio continuum emission. The black solid line represents the best-fit line determined by a least-squares polynomial fit with one degree to the data of the previous radio continuum surveys (black symbols) and this mm-RRL surveys (red symbols).