Provided is a method that includes a method for updating index data. The method includes receiving index data, including an index value indicative of user activity on a network site and an index time corresponding to a time used for calculating the index value, receiving an update index time corresponding to a time used for updating the index data, determining an updated index value using an exponential decay of the index value from the index time to the update index time, wherein the updated index value comprises a decayed value of the index value corresponding to the update time, and storing updated index data including the updated index value and the update index time.