1. A method for predicting risk of sudden cardiac death for a subject through analysis of surface electrocardiographic data recorded for the subject, the method comprising: processing a continuous surface electrocardiogram (EKG) signal received for the subject over a predetermined period of time to identify and extract a plurality of cardiac repolarization phase curves from the EKG signal in which each extracted cardiac repolarization phase curve represents an interval in the EKG signal extending between a corresponding feature of a respective cardiac cycle embodied in the EKG signal and the corresponding feature of a respective next sequential cardiac cycle embodied in the EKG signal; assigning each of the extracted cardiac repolarization phase curves into one of a plurality of groups based on a time duration of the cardiac repolarization phase curve between the corresponding feature of the respective cardiac cycle and the corresponding feature of the respective next sequential cardiac cycle; generating a respective graphical representation for each of the plurality of groups in which the cardiac repolarization phase curves assigned to the group are superimposed with one another in the respective graphical representation for the group; and evaluating the graphical representations generated for the plurality of groups to assess a risk of sudden cardiac death for the subject based on a classification of cardiac repolarization instability indicated by variations in voltage levels between the cardiac repolarization phase curves superimposed within the graphical representations.