This article statistically analyses how the citation impact of articles
deposited in the Condensed Matter section of the preprint server ArXiv (hosted
by Cornell University), and subsequently published in a scientific journal,
compares to that of articles in the same journal that were not deposited in
that archive. Its principal aim is to further illustrate and roughly estimate
the effect of two factors, 'early view' and 'quality bias', upon differences in
citation impact between these two sets of papers, using citation data from
Thomson Scientific's Web of Science. It presents estimates for a number of
journals in the field of condensed matter physics. In order to discriminate
between an 'open access' effect and an early view effect, longitudinal citation
data was analysed covering a time period as long as 7 years. Quality bias was
measured by calculating ArXiv citation impact differentials at the level of
individual authors publishing in a journal, taking into account co-authorship.
The analysis provided evidence of a strong quality bias and early view effect.
Correcting for these effects, there is in a sample of 6 condensed matter
physics journals studied in detail, no sign of a general 'open access
advantage' of papers deposited in ArXiv. The study does provide evidence that
ArXiv accelerates citation, due to the fact that that ArXiv makes papers
earlier available rather than that it makes papers freely available.