1. A computer-implemented method, comprising: receiving, by a computer system, a first set of information identifying an input set of documents, said input set comprising a plurality of documents; identifying, by the computer system, an additional document that is not a member of the input set, but which is citationally related to at least some of the documents in the input set; programmatically calculating, by the computer system, a data value that represents a degree to which said document is citationally related to the at least some of the documents in the input set, said data value dependent upon at least (a) how many citational relationships exist at generations higher than a first generation between the input set of documents and said additional document, and (b) generation levels of said citational relationships, wherein calculating said data value comprises assigning different amounts of weight to citational relationships of different generation levels, said amounts of weight being based at least in part on a generational citation count determined for each of the different generation levels and an analysis in which multi-generation citational relationships between documents are used to predict existences of first generation citational relationships between documents, said analysis performed over a document population; and storing the data value in computer storage in association with identifiers of the input set of documents and the additional document.