1. A computer-implemented method for determining an estimated demographic attribute value of a first online document, the method comprising: a) accepting, by a computer system including at least one computer, a set of one or more known values of a demographic attribute, each of the one or more known demographic attribute values being associated with an online document included in a plurality of second online documents, wherein each of the second online documents has a known value for the demographic attribute; b) accepting, by the computer system, tracked Internet surfing behavior of a plurality of independent users; c) analyzing, by the computer system and for each of the second online documents, the tracked Internet surfing behavior of the plurality of independent users to determine a probability that a user who has visited the second online document will visit or has visited the first online document, wherein the probability represents a single pair-wise relation between the second online document and the first online document; d) determining, by the computer system, the estimated demographic attribute value of the first online document using (i) the accepted known demographic attribute value of each of the second online documents and (ii) the determined probabilities; and e) associating, by the computer system, the demographic attribute and its determined estimated demographic attribute value with the first online document, wherein each of the plurality of second online documents is one of (A) a Website, or (B) a Web page for which the demographic attribute value of the demographic attribute is known, and wherein the first online document is one of (A) a Website, or (B) a Web page for which the estimated demographic attribute value of the demographic attribute is determined.