1. A method of improving segment combinations comprising information characterizing behaviors of groups of users of computers in a computer network, the method comprising: receiving, at a computer server, user response data concerning an advertisement exposed to a first plurality of computer users operating computers connected to the computer network, the identities of the computers of the users being identified with a segment combination based on the computers' previous accessing of a data publisher Website, the user response data providing a user response rate value indicative the responsiveness of the users of the computers to the advertisement; identifying from the user response data, those segment combinations within the user response data that have a user response rate surpassing a pre-determined value measured according to one or more user events; determining, for each permutation of permutations of the identified segment combinations, associated values indicative of statistical unification based on lift variations of the identified segment combinations; creating at least one segment combination as a new combination from the permutations of the identified segment combinations, wherein the resulting new combination has an associated value indicative of statistical unification that surpasses a threshold value and provides an indication of the behavioral similarity of the combined group in response to the advertisement; and writing an identifier of the new segment combination, using a computer server connected to the network, on computers of a second plurality of computer users at the time each second plurality computer user access a Web site of a data publisher to identify the computers of the second plurality users with the new combination, where the new segment combination is written to the computers to be accessible and identifiable to a serving system of an advertisement placement organization.