Line flux, ratio and S/N correlations from our CASA simulation. Upper panels: [C I](1−0) and CO(2−1) line brightness and ratio similar to Fig. 4, but from our simulation (with a constant input R_C I/CO = 0.2 as indicated by the horizontal shading in the right panels). Lower panels: Similar to the upper panels but showing line S/N instead of brightness. This illustrates how R_C I/CO can be biased in low-S/N situation.