Effect of the spherical prior on shape measures. We consider FoF halos of ΛCDM Universe simulated in a L = 648Mpc/h, N = 2048³ cube. Their prolaticity is computed: by diagonalizing the mass tensor of halos treated in three ways: through the procedure described in the text (substructures removal and isodensity selection) – this is the p₂₀₀ curve (orange); by simply ignoring particles out of the sphere whose spherical overdensity is Δ = 200. This boils down to introducing a spherical prior, and the corresponding curve is denoted p_S (green); or with no treatment on FoF halos – p_FoF (blue). It turns out that computing shapes of halos truncated by spheres considerably bias the measure of prolaticity (and of the other shape parameters), artificially bringing it closer to p = 0.