Radial abundance gradient for oxygen from observations. The data are taken by Deharveng et al. (2000) (filled blue squares), Rudolph et al. (2006) (filled green pentagon), Costa et al. (2004) (red filled triangles), Andrievsky et al. (2002a,b) (magenta filled hexagons), and Esteban et al. (2005) (black filled circles). We label with the large blue squared points the mean Cepheids values and relative error bars, whereas with the large black filled circles the mean values form the HII and planetary nebulae regions.