Probability distribution function of the linear polarization fraction (P_l) measured toward the 6.7 GHz CH₃OH maser features of the flux-limited sample that do not show (dashed red histogram; distribution d1) and show (solid black histogram; distribution d2) circular polarized emission. The interval width of the histograms is 0.5%. The data are taken from Surcis et al. (2009, 2011a, 2014b), Vlemmings et al. (2010), Papers I–IV, and this work.