Abstract | The effects of the parameter epistemic uncertainties on the predicted seismic performance are investigated for single-degree-of-freedom systems. The oscillators examined are 5%-damped systems having a complex quadrilinear backbone with pinching hysteresis, including an elastic, a hardening, a negative stiffness and a residual plateau branch, terminating with a final drop to zero strength. The six uncertain properties considered are the post-yield hardening ratio, the end-of-hardening ductility, the slope of the descending branch, the residual strength, the ultimate ductility and the period. Using Monte-Carlo simulation and the SPO2IDA tool for rapid performance estimation, we calculate the increased variability in the predicted limitstate capacities due to the epistemic uncertainty in each parameter. Thus, we are able to evaluate the influence of modeling assumptions to the predicted versus the actual seismic performance of the oscillator and understand the effect of the variability of each parameter to the total system variability at each limit-state.