Filled blue histogram: distribution of ⟨cos(Δ_S/V)⟩ for 10⁶ realizations of uniform Δ_S/V distributions for a sample size of 0.7 × 355. The red unfilled histogram corresponds to the values obtained for the data when removing the 30% of the sample with the higher values of the R_P/p ratio. This distribution takes into account the errors on the polarization position angles. The green unfilled histogram shows 10⁶ realizations of 30% random truncation of the original sample.