Abstract: Systems and techniques are disclosed for creating segments of users that include baseline users having specified traits and users that are similar to the baseline users. A segment is created by identifying baseline users based on a segment rule that specifies one or more traits of the users to include. The data about the baseline and other users in the dataset is used to extend the segment. A representation of the segment is determined, for example, by determining average values of numeric traits and frequencies of non-numeric trait values of the baseline users in the segment. The representation of the segment is used to determine the similarity (i.e., similarity scores) of users to the segment and ultimately to determine which of the other users, who are not already included in the segment, should be included in the segment based the similarity of their traits to those of the segment representation.