Panel a: iron radial gradient for classical Cepheids as a function of the Galactocentric distance. The linear, the logarithmic and the bilinear fits are shown respectively in blue, red and green. The vertical green dashed line shows the ‘knee’ of the bilinear fit. The coefficients of the fits are labelled. Panel b: residuals of the linear fit. The values of the mean square error, the reduced chi-squared and the AIC score are labelled. Panel c: same as panel b, but for the logarithmic fit. Panel d: same as panel b, but for the bilinear fit.