As Fig. 9, but comparing our default τ(250 μm) /τ(J) estimates (red solid circles) and results from alternative analyses: τ(250 μm estimates derived assuming a spatially varying spectral index (crosses), τ(250 μm bias estimated with Ossenkopf & Henning (1994) dust model (open circles), τ(J) bias estimated using k = 1.0 × 10^-3 (triangles pointing upwards), and τ(J) bias estimated using k = 3.0 × 10^-3 (triangles pointing downwards).