An exemplary method includes selecting a cross-correlation frequency having an associated cross-correlation period, detecting and binning a heart rate in a heart rate bin, detecting and binning an activity state in an activity state bin, repeating the detecting and binning a heart rate and the detecting and binning an activity state during a cross-correlation period, and summing the products a bin count of the heart rate bins and a bin count of the activity state bins to provide a cross-correlation index for the cross-correlation period. Other exemplary algorithms, methods, devices, systems, etc., are also disclosed.