We provide a method to correct the observed azimuthal anisotropy in heavy-ion collisions for the event plane resolution in a wide centrality bin. This new procedure is especially useful for rare particles, such as Omega baryons and J/psi mesons, which are difficult to measure in small intervals of centrality. Based on a Monte Carlo calculation with simulated v_2 and multiplicity, we show that some of the commonly used methods have a bias of up to 15%.