An apparatus and method for determining a signal quality of an input signal representing a repetitious phenomena derived from at least one sensor connected to a patient is provided. A detector receives the input signal and determines data representing the repetitious phenomena from the input signal for use in determining at least one patient parameter. A measurement processor is electrically coupled to the detector that determines a first signal quality value by identifying at least one feature of the repetitious phenomena data and compares the at least one feature of a first set of the determined repetitious phenomena data with a second set of the determined repetitious phenomena data to determine a feature variability value and using the feature variability value to determine a stability value representative of the quality of the input signal.