Coefficients C₀ (panel a), C_SB (panel b), C_h (panel c, circles), and C_T (panel c, squares) in parametric OH–SB relation as a function of galactocentric distance. The dashed lines are the best fits to the data points. The dotted lines are the corresponding coefficients in the simple OH–SB relation. Panel d shows the deviations around the simple relation, residuals of Equation (18), (crosses) and around the parametric relation, residuals of Equation (19), (circles) as a function of galactocentric distance.