Comparison of joint marginal and conditional distributions for different data types. The contour lines indicate highest density regions containing (from dark to light): 10 per cent, 25 per cent, 50 per cent and 75 per cent of the total probability mass. The first row shows the joint marginal distributions of the viscosity η_w in the hinge zone and density ρ₂ of the subducting plate, where the columns indicate the different data types used: (a) only plate velocities, (b) only normal traction and (c) both plate velocities and normal traction. The second row (d)–(f) depicts the conditional distributions of viscosity and density given the true shape of the subducting plate.