It is certainly welcome news that there are no impact differences between the 191 OA journals and the 8509 non-OA journals indexed by ISI, equating for comparable journals as closely as possible: http://www.isinet.com/oaj. This proves that the skeptics who thought OA journals would be of lower quality or impact and would not be indexed by ISI were wrong (at least for those OA journals): OA journals are indexed by ISI, and they do have comparable citation impacts. But obviously this methodology has a certain element of circularity!
To get a realistic estimate of the effect of OA on impact, it is not enough to compare only the 2% of ISI journals that are OA journals with the 98% that are not, to find that they are equal in impact (for this may well be comparing apples with oranges, even if you equate for subject matter).
What further needs to be compared is:
(1) the citation impact of the much
higher percentage (perhaps as high as 20-40% according to Swan
& Brown's (2004) sample) of articles
from the 98% non-OA journals that
have been made OA by
their authors (by self-archiving
(2) the citation impact of articles from
those very same journals and issues that have not been made OA by their
As the founder of citation analysis as well as the ISI, Gene
Garfield (1998) has often stressed, the article (and author)
citation counts need to be analyzed, not just the average
citation counts of the journals in which the article appears.
What this kind of analysis is beginning to reveal in the OA era is that there is indeed a discernible difference in terms of the frequency with which the article is cited: there is a dramatic advantage in favor of the articles that their authors have made OA (Lawrence 2001; Kurtz 2004; Brody et al. 2004). Results are only available for computer science, astronomy, and physics so far, but all other disciplines are currently being analyzed.The earlier Lawrence (2001) study on the impact enhancing effects of OA in computer science needed to be replicated in other fields to check whether it was merely an artifact of the fact that computer science is conference- rather than journal-based, and whether the advantage really reflected OA vs. non-OA rather than just online access vs. paper access. But, thanks to the ISI database licensed to the Observatoire des Sciences et des Technologies (OST) and a special contract generously provided by ISI to conduct the study, we are in the process of testing the Lawrence effect across all disciplines in a 10-year ISI sample of 14 million articles. The physics analyses up to 2001 are already completed (Brody et al. 2004), and they reveal even larger effects than those reported by Lawrence, with OA/non-OA citation ratios of 2.5 - 5.8. All indications are that the 2002 data will raise them even further, as the biggest effects occur within the first 3 years of publication in scientific disciplines (and both OA and the awareness and visibility of OA articles are also increasing yearly).
Access is not a sufficient condition for citation, but it is a
necessary one. Because Open Access (OA) dramatically
increases the number of potential users of any given article by adding
those who would otherwise have
been unable to use it because their institution could not afford the
access-tolls of the journal in which it appeared, it stands to reason
that OA can only
increase both usage and impact. The ratio of "reads" to "cites" will no
doubt vary by field. Kurtz
(2004) and co-workers report it as 17:1 and even 12:1 in
(2002) predicts analogous trends in mathematics. Tim Brody's
remarkable correlator/predictor gives
the size of the download/citation correlation by field, and can be used
citation 6-24 months later from downloads today (with an adjustable
The proportion of articles for which their authors provide OA is
likely to increase dramatically now, in part because of the mounting
evidence for the impact advantage OA confers. OA will also increase
because of the growing number of journals that have already given their
official "green light" to author self-archiving, partly because journal
impact factors too benefit from increased article impact, and partly
because journals are eager to demonstrate that they have no wish to
stand in the way of OA and its benefits to research and researchers.
(The proportion of "green" journals rose from 55% to 83% from
A JISC survey (Swan
& Brown 2004):
It is hence clear that the ball is now in the the universities'
court: The sooner they extend their existing publish-or-perish policies
to require also providing OA for all those published articles, the
sooner the entire research community will enjoy the benefits of
maximizing its research impact by maximizing access to its research: http://www.eprints.org/signup/sign.php