  We report characteristics of in-text citations in over five million full text
articles from two large databases - the PubMed Central Open Access subset and
Elsevier journals - as functions of time, textual progression, and scientific
field. The purpose of this study is to understand the characteristics of
in-text citations in a detailed way prior to pursuing other studies focused on
answering more substantive research questions. As such, we have analyzed
in-text citations in several ways and report many findings here. Perhaps most
significantly, we find that there are large field-level differences that are
reflected in position within the text, citation interval (or reference age),
and citation counts of references. In general, the fields of Biomedical and
Health Sciences, Life and Earth Sciences, and Physical Sciences and Engineering
have similar reference distributions, although they vary in their specifics.
The two remaining fields, Mathematics and Computer Science and Social Science
and Humanities, have different reference distributions from the other three
fields and between themselves. We also show that in all fields the numbers of
sentences, references, and in-text mentions per article have increased over
time, and that there are field-level and temporal differences in the numbers of
in-text mentions per reference. A final finding is that references mentioned
only once tend to be much more highly cited than those mentioned multiple
times.
