The so-called tracking deviation refers to the deviation between the return rate of index funds and the return rate of the underlying index. Tracking deviation describes the closeness between the fund and the underlying index according to the historical difference data of the return rate, and reveals the fluctuation characteristics of the fund return rate around the underlying index return rate. Generally speaking, the accuracy of tracking deviation is related to the length of observation period. The longer the observation period and the more observation points, the more accurate the calculated tracking deviation will be. Tracking deviation is a risk indicator, which reflects the deviation between portfolio return and target return. The greater the tracking deviation, the greater the deviation from the target, the higher the risk, the smaller the tracking error, and the smaller the deviation from the tracking target, the lower the risk.