The Robust Performance Number (RPN) and the Robust Performance Number of a Plant Set (RPPN) ate introduced as measures to determine the controllability of a system. These numbers indicate how potentially difficult it is for a given system to achieve the desired performance robustly. The RPN and the RPPN reflect both the attainable performance of a system and its degree of directionality. The RPN is used for the analysis of the nominal system, whereas the RPPN can be used to include the effects of the nonlinearities and uncertainties of the model. It is shown how the attainable performance can be determined. As appropriate scaling is crucial for the analysis and for the design of multivariable systems, a strategy for system scaling based on the RPN is also presented.