There are natural clusters. It's always possible to find fringe cases where the rules don't really hold, or cases on the boundary. That does not affect the basic validity of citation analysis, any more than such problems affect the validity of other scientific approaches. . There are always small differences, and I can discuss at some length whether, for example, Journal of Biological Chemistry and Biochemistry (ACS) are in separate microclusters. But the same basic citation patterns hold in both of them. When I collected at Princeton, I purchased for the biology library everything about intelligent design having any reference to the ordinary scientific literature, on the grounds that the biologists need to know about it. There is actually not all that much cross-citation: the ID people cite a very small part of the biology literature, and only to attack it. (And the biologists n turn cite a very small part of the fundamentalist religious material) That pattern pretty much holds in the only fringe and pseudo sciences--they don't really talk to theregular sciences and vice versa. And there are good examples of work done on ostensibly the same subject where there are isolated literatures--psychoanalysis vs. the rest of psychiatry & psychology is a good example--one I used for teaching. Medline covers both, but there are very few cross citations David Goodman, Ph.D., M.L.S. dgoodman@princeton.edu