Probability density funtion of the linear polarization fraction (P_l, left panel), of the circular polarization fraction (P_V, middle panel), and of the Zeeman-splitting (ΔV_Z, right panel) of the 6.7 GHz CH₃OH maser emission. The interval width of the histograms is 0.5% and 0.25% and 0.25 m s⁻¹ for the P_l, P_V, and the ΔV_Z plots, respectively. The data are taken from: Surcis et al. (2009, 2011a, 2014b), Vlemmings et al. (2010), Papers I–IV, and this work.