Panel a) shows the CO depletion factor (from ) as a function of N(H₂). Each group of sources is shown with a different symbol and colour, as indicated. Panel b) is the same as a), but for f_D as a function of n(H₂). Panel c) shows that removing the T_ex-dependence the trends are still visible (see text). The solid lines are simple non-parametric regressions made using the R (R Core Team 2014) np package (Hayfield & Racine 2008) (excluding the outlier indicated in red); bootstrapped errors are indicated. Symbol sizes are proportional to the distance of the source.