A method for determining a physiological parameter in the presence of correlated artifact, including obtaining two digital waveforms, x and y, the waveforms being representative of the absorption of two wavelengths of electromagnetic energy received from a blood-perfused tissue, and where each of the waveforms has a component corresponding to a plethysmographic waveform and a component corresponding to the correlated artifact; calculating several weighted difference waveforms of the form x-R*y, where R is a multiplier, by varying R over a range; evaluating the several weighted difference waveforms using a shape characteristic of the weighted difference waveform; identifying a weighted difference waveform most closely representative of and one most different from the plethysmographic waveform; determining a pleth-based physiological parameter using the waveform most closely representative of the plethysmographic waveform; determining at least one artifact-based physiological parameter using the waveform most different from the plethysmographic waveform; and rejecting other possible candidate values for the pleth-based physiological parameter using the artifact-based physiological parameter.