The contribution deals with the determination of the curves of the annual number of lightning induced flashover vs. the critical impulse flashover voltage of the insulators, used by power engineers for the assessment of the lightning performance of distribution lines. The results obtained with the method proposed by the authors are compared with those dealt with in the IEEE 1410-1997 Guide (distribution lightning performance). The two methods differ for (a) the models adopted for the evaluation of the induced voltages and (b) for the adopted statistical approach. Reasons for differences in the results predicted by the two methods are illustrated and justified, and the parameters that play the major role in the inference of the curves are identified.