Abstract :
Statistical relationships between downloads from
ScienceDirect of documents in Elsevier’s electronic
journal Tetrahedron Letters and citations to these documents
recorded in journals processed by the Institute for
Scientific Information/Thomson Scientific for the Science
Citation Index (SCI ) are examined. A synchronous
approach revealed that downloads and citations show
different patterns of obsolescence of the used materials.
The former can be adequately described by a model consisting
of the sum of two negative exponential functions,
representing an ephemeral and a residual factor,
whereas the decline phase of the latter conforms to a
simple exponential function with a decay constant statistically
similar to that of the downloads residual factor.
A diachronous approach showed that, as a cohort of
documents grows older, its download distribution becomes
more and more skewed, and more statistically
similar to its citation distribution. A method is proposed
to estimate the effect of citations upon downloads using
obsolescence patterns. It was found that during the first
3 months after an article is cited, its number of downloads
increased 25% compared to what one would expect
this number to be if the article had not been cited.
Moreover, more downloads of citing documents led to
more downloads of the cited article through the citation.
An analysis of 1,190 papers in the journal during a time
interval of 2 years after publication date revealed that
there is about one citation for every 100 downloads. A
Spearman rank correlation coefficient of 0.22 was found
between the number of times an article was downloaded
and its citation rate recorded in the SCI. When initial
downloads—defined as downloads made during the first
3 months after publication—were discarded, the correlation
raised to 0.35. However, both outcomes measure the
joint effect of downloads upon citation and that of
citation upon downloads. Correlating initial downloads
to later citation counts, the correlation coefficient drops
to 0.11. Findings suggest that initial downloads and citations
relate to distinct phases in the process of collecting
and processing relevant scientific information that
eventually leads to the publication of a journal article.
