Impact of the effective ellipticity of the covariance matrix on the statistical relative Bias_p (left column) and Risk_p (right column) quantities in the extreme (light shaded region) and low (dark shaded regions) regimes, for both MAS (orange, top) and MB (pink, bottom) estimators. The domain of the naïve measurements is repeated in grey shaded regions on both plots. The canonical case of the MAS (and MB) is also repeated on each panel in dashed orange (and pink) lines. This is based on 100 000 Monte-Carlo simulations for each set-up of the covariance matrix, the S/N, and the true polarization parameters.