  Eysenbach has suggested that the OA (Green) self-archiving advantage might
just be an artifact of potential uncontrolled confounding factors such as
article age (older articles may be both more cited and more likely to be
self-archived), number of authors (articles with more authors might be more
cited and more self-archived), subject matter (the subjects that are cited
more, self-archive more), country (same thing), number of authors, citation
counts of authors, etc. Chawki Hajjem (doctoral candidate, UQaM) had already
shown that the OA advantage was present in all cases when articles were
analysed separately by age, subject matter or country. He has now done a
multiple regression analysis jointly testing (1) article age, (2) journal
impact factor, (3) number of authors, and (4) OA self-archiving as separate
factors for 442,750 articles in 576 (biomedical) journals across 11 years, and
has shown that each of the four factors contributes an independent,
statistically significant increment to the citation counts. The
OA-self-archiving advantage remains a robust, independent factor. Having
successfully responded to his challenge, we now challenge Eysenbach to
demonstrate -- by testing a sufficiently broad and representative sample of
journals at all levels of the journal quality, visibility and prestige
hierarchy -- that his finding of a citation advantage for Gold OA (articles
published OA on the high-profile website of the only journal he tested (PNAS)
over Green OA articles in the same journal (self-archived on the author's
website) was not just an artifact of having tested only one very high-profile
journal.
