Relationships between ice column density and visual extinction (A_V) as observed toward stars behind dense clouds and cores for H₂O (black), CO (green), CO₂ (red), and CH₃OH (blue). For clarity, for H₂O, CO, and CO₂, the least-squares linear fits are shown and not the actual data points. For CH₃ OH, blue bullets indicate detections with 1-σ error bars and open circles indicate 3-σ upper limits. The solid blue line is a fit to the detections, and the dashed blue line (from exp. 1.0) indicates the scenario in which CH₃OH has the same ice formation threshold as H₂O at a maximum abundance of CH₃OH:H₂O = 4%, as found in the listed experiments. The 3 σ upper limits prove in principle that CH₃OH:H₂O is less than 4% in the observed sight lines. This figure was adapted from Fig. 7 in Boogert et al. (2015).