The present disclosure describes the derivation and measurement of a fractional oxygen saturation measurement. In one embodiment, a system includes an optical sensor and a processor. The optical sensor can emit light of multiple wavelengths directed at a measurement site of tissue of a patient, detect the light after attenuation by the tissue, and produce a signal representative of the detected light after attenuation. The processor can receive the signal representative of the detected light after attenuation and determine, using the signal, a fractional oxygen saturation measurement based on two or more different measures of fractional oxygen saturation.
