Panel a shows deviations of the central oxygen abundances from the one-dimensional O/H–SB relation (the residuals of Equation (7)) as a function of the uncertainty in central oxygen abundance. Panels b shows deviations of the oxygen abundances at the isophotal R₂₅ radius of a galaxy from the one-dimensional O/H–SB relation (the residuals of Equation (12)) as a function of the uncertainty in the oxygen abundances. The points show the values of the individual galaxies. The dotted lines indicate residuals of zero.