  Scientific abstracts contain what is considered by the author(s) as
information that best describe documents' content. They represent a compressed
view of the informational content of a document and allow readers to evaluate
the relevance of the document to a particular information need. However, little
is known on their composition. This paper contributes to the understanding of
the structure of abstracts, by comparing similarity between scientific
abstracts and the text content of research articles. More specifically, using
sentence-based similarity metrics, we quantify the phenomenon of text re-use in
abstracts and examine the positions of the sentences that are similar to
sentences in abstracts in the IMRaD structure (Introduction, Methods, Results
and Discussion), using a corpus of over 85,000 research articles published in
the seven PLOS journals. We provide evidence that 84% of abstract have at least
one sentence in common with the body of the article. Our results also show that
the sections of the paper from which abstract sentence are taken are invariant
across the PLOS journals, with sentences mainly coming from the beginning of
the introduction and the end of the conclusion.
