Histogram of the 355 relative angles Δ_S/V computed as in Eq. (2). The observed distribution is significantly not uniform and shows an excess toward 90°, which corroborates a contamination scenario. The central values and error bars given in red account for observational uncertainties. The errors are estimated through 10⁶ simulations in which we vary the polarization position angles of the quasars and dust around their values according to the observational uncertainties (see text for details). The central values and error bars correspond to the mean and dispersion in each bin obtained by computing the histogram for each simulation. The average correlation appears weaker because of the blurring of the correlation that the randomization of the angles within their uncertainties produces.