1. A subject monitoring system for processing physiological information of a subject, comprising: a sensor configured to generate a physiological 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 skew metric value based on the physiological signal; determine a correlation lag value corresponding to a peak in a correlation sequence derived from the physiological signal; and qualify or disqualify the correlation lag value based on the skew metric.