Correlation between the gas mass estimated (or “measured”) from a combination of the [O i] 63 μm line flux and the [O i] 63/CO 2−1 line ratio and the input gas mass of the disc. The blue dots are the mean “measured” gas masses and the error bar denotes the standard deviation. The red solid line is the expected correlation together with the dashed lines that denote an uncertainty of one order of magnitude.