1. A computer-implemented method for determining a demographic attribute value of a sink online document, the method comprising: a) accepting a set of one or more values of the demographic attribute, each of the one or more demographic attribute values being associated with a source online document, wherein each of the source online documents has a value for the demographic attribute; b) accepting, for each of the source online documents, a probability that a user will visit or has visited the sink online document if the user visited the source online document; and c) determining the demographic attribute value of the sink online document using (i) the accepted demographic attribute value of each of the source online documents and (ii) the accepted probabilities.