Since worst case response times must be determined repeatedly during the interactive design of real-time application systems, repeated exact computation of such response times would slow down the design process considerably. In this research, we identify three desirable properties of estimates of the exact response times: continuity with respect to system parameters, efficient computability, and approximability. We derive a technique possessing these properties for estimating the worst-case response time of sporadic task systems that are scheduled using fixed priorities upon a preemptive uniprocessor