Summary form only given. In this paper, qualitative and quantitative differences between nonlinear and linear modal simulations in stressed power system are presented. For the first time in the literature, time-domain nonlinear simulation is used to validate the accuracy of the second order modal model, obtained by using modal series. Furthermore three new selective indices are defined and used to explain and predict the differences between nonlinear and linear modal simulations. These indices also explain the under and over dumping results being experienced when a linear system theory designed controller is applied to a nonlinear system.