Analytical results relating errors in transfer-function coefficients for a second-order model to errors in the resulting time-domain response, via the integral of the squared error (I.S.E.), are presented. Such information is valuable in assessing the application of transfer-function approximations, such as the continued fraction expansion. The nature of the transfer function, i.e. overdamped or underdamped, has a significant effect on the time-domain I.S.E. Both impulse and step responses are considered.