Radial abundance gradient for oxygen. The black solid line refers to one infall model without threshold and with the inside-out formation (S1I). The red short-dashed line represents the model with a radial inflow of 1 km s^-1 (R1I1) using shells 2 kpc wide and the blue long-dashed line label the same physical parameters but running a model with shells 1 kpc wide. The data are mean values and relative errors from Cepheids by Andriesky et al. (2005a,b).