Bibliometrics can be defined as a field of research that examines bodies of knowledge both within and across disciplines. Although many methods are commonly used, perhaps the most widely known is citation analysis; that is, tracking published articles to see whether they are subsequently cited by others. Much of contemporary bibliometrics can be traced back to a seminal publication known as Shepard's Citations, a tool first used by American lawyers in 1873 to establish whether a previous legal judgment had been referred to, overruled, or made invalid in some other way. By the early 20th century, citation analysis had attracted the attention of various scientific scholars, although most of their early work simply involved the counting and sorting of reference lists. Nevertheless, some trends were noticed early on in the journals of chemistry, engineering and physiology. Perhaps the most striking observation was that not all journals were being equally cited; rather, only a few core periodicals appeared to be attracting the majority of all citations. On the other hand, while larger journals tended to gather more citations than smaller ones, some of the smaller periodicals still appeared to be performing well, relative to their actual size and circulation.