Harmonic voltages at PCC of a power grid are the combinations of voltage contributions of each harmonic source located in the same grid. In particular, quantifying harmonic contributions of the customers and the utility system is highly important for power quality management. This paper presents a new method of harmonic source identification problems: how to quantify the harmonic levels observed at a network location. The paper first defines the problem. It then presents a statistical correlation based method to estimate the index. The characteristics of the proposed method are investigated through case studies. Finally, the improvements of the proposed method are discussed.