Statistics of polarization fractions in the simulated Planck observations as a function of viewing angle α (see Fig. 13). The solid blue line shows p_max, the solid black line shows the mean p, and the solid green line shows the value of p for the most diffuse lines of sight in the map. The dashed black line marks the polarization fraction parameter p₀ and the dashed green line gives the theoretical polarization fraction in the case where the density and magnetic field are homogeneous and the latter makes an angle α with the plane of the sky (see text). The grey region shows the ± 1σ spread around the mean p.