1. A method of classifying user activities, comprising: at a computer system having one or more processors and memory: receiving eye tracking data for a person viewing a page having a plurality of distinct contiguous regions, the eye tracking data comprising a sequence of fixations ordered temporally, wherein each respective fixation corresponds to a plurality of respective measured gaze points, and wherein each respective fixation has a respective duration and location based on the corresponding respective measured gaze points; partitioning the fixations into a plurality of clusters, wherein each cluster comprises a consecutive sub-sequence of the fixations in the temporal order; assigning a provisional user activity label to each fixation based on a set of characteristics of the fixation, wherein the user activity labels are selected from a predefined set of user activity labels; grouping together consecutive fixations that have the same assigned provisional user activity label to partition the fixations into a plurality of groups; for each respective group that matches a respective cluster, retaining the provisional user activity label assignment as a final user activity label assigned to each of the fixations in the respective group; and reconciling non-matching groups with non-matching clusters, using the regions, to form a set of non-overlapping modified groups consisting of fixations from the non-matching groups, wherein each fixation in a modified group is assigned a same final user activity label selected from the predefined set of user activity labels.