Column densities of N₂D⁺ compared to those of N₂H⁺ and obtained under (a) LTE and (b) NLTE conditions. The different colours of markers refer to the evolutionary stages of clumps, as in Fig. 1. Lines show the median values of the deuteration fraction for the quiescent group (solid line; $R_{\text{D}}^{\text{LTE}}=0.079\pm 0.029$ and $R_{\text{D}}^{\text{NLTE}}=0.014\pm 0.005)$), the protostellar group (dashed line; $R_{\text{D}}^{\text{LTE}}=0.025\pm 0.006$ and $R_{\text{D}}^{\text{NLTE}}=0.006\pm 0.001$), and the YSO group (dotted line; $R_{\text{D}}^{\text{LTE}}=0.011\pm 0.004$ and $R_{\text{D}}^{\text{NLTE}}=0.004\pm 0.002$). The black bars show the uncertainties associated with N^LTE , while the grey bars refer to the ranges in N^NLTE listed in Table A.2.