Bias on α_Measured as a function of α_Input (from lognormal simulations) for Landy-Szalay (blue points), iterative optimal (red triangles) and two non-iterative minimum-variance estimators with α = 0.96 (orange crosses) and α = 1.04 (green squares). The error bars indicate the rms among the lognormal simulations. Simple linear functions were fitted to the points in each case with corresponding slopes shown in the legend. A strong bias can be seen for the non-iterative estimators while the iterative optimal and Landy-Szalay estimators are not biased. (Coloured version of the figure available online.)