Panels one to four, from top: comparison between the mean deuterated fractions (black dots) of N₂H⁺ (first panel, Paper I), HNC (second panel, Paper II) NH₃ and CH₃OH (third and fourth panels, this work). The mean values have been computed for each evolutionary group. Quiescent HMSCs (HMSC-q) and warm HMSCs (HMSC-w) have been treated separately. The error bars indicate the standard deviation. The grey arrows represent mean upper limits for those evolutionary groups in which no sources have been detected. The red dot in the fourth panel represents the doubtful CH₂DOH detection in G034-G2 (see Sect. 4.4), while the square indicates the mean D_frac of HMPOs when N(CH₂DOH) is derived from rotation diagrams for AFGL5142–MM and 18089–732. Bottom panel: mean rotation temperatures (filled diamonds) derived from ammonia in the four groups (see Table 7).