Design of robust controllers in the face of process model uncertainty is an important problem. The new design methodologies in the frequency domain have sought to address some of the issues involved in robust control. Nevertheless, the characterization and measurement of uncertainty remains a somewhat difficult and abstract task. In order to easily compare and characterize robust performance of alternate stable control structures, it would be useful to have an intuitive, easily applicable measure of robust performance. In this paper we propose a new time domain based measure of robust performance using geometric concepts borrowed from Response Surface Methodology. The measure is used to characterize robust performance of single-input-single-output systems. Simulations results are provided to illustrate the use and intuitive appeal of the new measure.