1. A subject monitoring system for determining physiological information of a subject, comprising: a sensor configured to generate an intensity signal, wherein the sensor detects light attenuated by the subject; and processing equipment coupled to the sensor, wherein the processing equipment is configured to: determine a value indicative of a physiological rate of the subject; perform a cross-correlation sequence based on the intensity signal and a reference waveform; select two segments from the cross-correlation sequence; and determine whether to qualify the value indicative of the physiological rate of the subject based on the two segments.