Comparison of the predicted molecular gas mass estimates with the true molecular gas mass using two different approaches for simulated galaxies at z = 4. Panel a shows the performance of the best-fit relation between M_mol and L_[C II], while panels b–d present results from different PCA-based relations (as indicated in the title of each panel). The corresponding relations are listed in Table 5. In panels a–d, the top-left corner reports σ, the standard deviation of the offset between the predicted and true M_mol (both in log scale). Panel e compares the distributions of these offsets from the different approaches.