Spread in deuteration fractions D_frac in the four evolutionary stages for HCN, HNC, HCO⁺, and N₂H⁺. If there are six or more detections within one stage, the blue box shows the 25−75% range and the crosses mark the other 50% of the data points. If there are fewer than six data points, only the individual data points are shown. The red solid line shows the median of all detections and upper limits. The downward arrow shows the highest value of all upper limits within that stage. The fractions at the bottom of each stage give the detection rate.