Many factors affect the seismic response of a structural system. A study was carried out to determine the effect of different combinations of damping on the predicted response of secondary and tertiary subsystems. To simplify the results and to allow for easier interpretation, two- and three-degree-of-freedom structural systems were investigated. The study involved determining the "best estimate" seismic responses for 12 historic earthquake time histories and comparing them to the predicted responses obtained by using two simplified methods of analysis which have been in common use. Curves have been developed which specify the expected average error for the simplified methods by mass ratio(s) and damping. These results provide an overview as to the ranges of parameters where upper level interaction and nonproportional damping effects are significant. Key words: nonproportional damping, complex modal analysis, dynamic, seismic, equipment, tertiary subsystems.