log-log representation of the global error for the Stokes vector components I, Q, U and V as a function of the number of points-per-decade of continuum optical depth for different formal solvers (color coded). The Sr I line at 4607.3 Å is considered for the smooth FALC atmospheric model (first row), and for three additional atmospheric models that contain discontinuities in temperature (second row), in line of sight velocity (third row), and in magnetic field (fourth row). The atmospheric models are described in Fig. 3. The global error is computed as exposed in Eq. (A.1).