Cumulative distributions of dust masses in our model (left panel) and dust masses estimated from ALMA observations (right panel). Thin lines in the left panel show the true disc mass and the thick lines show the optically thin disc mass at λ = 1.3 mm. At most times, the differences between the two are small as discs are marginally or fully optically thin. The exception to this are the very young and massive discs, or old discs which have retained dust outside a photoevaporative gap.