Dependence of the parameters of the derived oxygen abundance distribution in NGC 3521 on the positions of the centres of bins. Panela: Mean value of the deviations of the binned oxygen abundance around the obtained broken O/H – R relation as a function of the position of the centre of the first bin. Panelb: Derived value of the central oxygen abundance as a function of the position of the first bin centre. The bars show the uncertainties of the obtained values. Panelc: Same as panel (b), but for the abundances at the isophotal radius. Paneld: Value of the break radius as a function of the position of the centre of the first bin. The circles denote the break radius in O/H – R distribution, and the crosses mark the break radius in N/H – R distrinutions. The bars show the uncertainties of the obtained values.