Polarization degree distribution for the observations in 2008, 2009, 2011, and 2012 when considering the null hypothesis (p = 0%, φ = 0°), the observed flux density I, and its uncertainty. The most probable observed polarization degree, p_obs in each case is the mode of the distributions. The range that contains 68% of the most probably observed values, 1σp_obs is delineated with vertical dashed lines. The smooth line over the histograms corresponds to the best-fit Rice function.