A method of quantifying the relative stability of rotordynamic oscillations is developed. The approach is based upon application of Floquet theory, in conjunction with Poincaré mapping, to measured (and periodic) vibration data. Detailed knowledge of a dynamic model or of the internal dynamics of the system is not required. Example cases are evaluated for a rotor interacting with a clearance bearing in order to demonstrate the method. Results are presented and discussed for simulation data (obtained from a simplified model for the example system) and for measured data from an experimental test rig.