Posterior probability density functions B_2D(p₀,ψ₀ | p,ψ,Σ_p) computed for the most probable measurements (p, ψ) of the f_2D distribution (crosses), which were obtained for a given set of true polarization parameters ψ₀ = 0° and p₀ = 0.10 (dashed lines) and various configurations of the covariance matrix, at four levels of S/N p₀/σ_p,G = 0.1,0.5,1, and 5 (top to bottom). The scales of the p₀ and ψ₀ axes may vary from one row to the next in order to focus on the interesting part of the PDF. The black contours provide the 90, 70, 50, 20, 10, 5, 1, and 0.1% levels.