Michael Kurtz's papers have confirmed that in astronomy/astrophysics (astro), articles that have been self-archived -- let's call this "Arxived" to mark it as the special case of depositing in the central Physics Arxiv -- are cited (and downloaded) twice as much as non-Arxived articles. Let's call this the "Arxiv Advantage" (AA).
SUMMARY: In astrophysics, Kurtz found that articles that were self-archived by their authors in Arxiv were downloaded and cited twice as much as those that were not. He traced this enhanced citation impact to two factors: (1) Early Access (EA): The self-archived preprint was accessible earlier than the publisher's version (which is accessible to all research-active astrophysicists as soon as it is published, thanks to Kurtz's ADS system). (Hajjem, however, found that in other fields, which self-archive only published postprints and do have accessibility/affordability problems with the publisher's version, self-archived articles still have enhanced citation impact.) Kurtz's second factor was: (2) Quality Bias (QB), a selective tendency for higher quality articles to be preferentially self-archived by their authors, as inferred from the fact that the proportion of self-archived articles turns out to be higher among the more highly cited articles. (The very same finding is of course equally interpretable as (3) Quality Advantage (QA), a tendency for higher quality articles to benefit more than lower quality articles from being self-archived.) In condensed-matter physics, Moed has confirmed that the impact advantage occurs early (within 1-3 years of publication). After article-age is adjusted to reflect the date of deposit rather than the date of publication, the enhanced impact of self-archived articles is again interpretable as QB, with articles by more highly cited authors (based only on their non-archived articles) tending to be self-archived more. (But since the citation counts for authors and for their articles are correlated, one would expect much the same outcome from QA too.) The only way to test QA vs. QB is to compare the impact of self-selected self-archiving with mandated self-archiving (and no self-archiving). (The outcome is likely to be that both QA and QB contribute, along with EA, to the impact advantage.)
Henneken, E. A., Kurtz, M. J., Eichhorn, G., Accomazzi, A., Grant, C., Thompson, D., and Murray, S. S. (2006) Effect of E-printing on Citation Rates in Astronomy and Physics. Journal of Electronic Publishing, Vol. 9, No. 2, Summer 2006Kurtz analyzed AA and found that it consisted of at least 2 components:
Henneken, E. A., Kurtz, M. J., Warner, S., Ginsparg, P., Eichhorn, G., Accomazzi, A., Grant, C. S., Thompson, D., Bohlen, E. and Murray, S. S. (2006) E-prints and Journal Articles in Astronomy: a Productive Co-existence (submitted to Learned Publishing)
Kurtz, M. J., Eichhorn, G., Accomazzi, A., Grant, C. S., Demleitner, M., Murray, S. S. (2005) The Effect of Use and Access on Citations. Information Processing and Management, 41 (6): 1395-1402, December 2005
Moed, H. F. (2006, preprint) The effect of 'Open Access' upon citation impact: An analysis of ArXiv's Condensed Matter SectionMoed too has shown that in cond-mat the AA effect (which he calls CID "Citation Impact Differential") occurs early (1-3 years) rather than late (4-6 years), and that there is more Arxiving by authors of higher-quality (based on higher citation counts for their non-Arxived articles) than by lower-quality authors. But this too is just as readily interpretable as the result of QB or QA (or both): We would of course expect a high correlation between an author's individual articles' citation counts and the author's average citation count, whether the author's citation count is based on Arxived or non-Arxived articles. These are not independent variables.
Davis, P. M. and Fromerth, M. J. (2007) Does the arXiv lead to higher citations and reduced publisher downloads for mathematics articles? Scientometics, accepted for publication. See critiques: 1, 2.
Harnad, S. (2005) OA Impact Advantage = EA + (AA) + (QB) + QA + (CA) + UA. Preprint.Moed's analysis, like Kurtz's, cannot decide between QB and QA. The fact that most of the AA comes in an article's first 3 years rather than its second 3 years simply shows that both astro and cond-mat are fast-developing fields. The fact that highly-cited articles (Kurtz) and articles by highly-cited authors (Moed) are more likely to be Arxived certainly does not settle the question of cause and effect: It is just as likely that better articles benefit more from Arxiving (QA) as that better authors/articles tend to Arxive/be-Arxived more (QB).
Hajjem, C., Harnad, S. and Gingras, Y. (2005) Ten-Year Cross-Disciplinary Comparison of the Growth of Open Access and How it Increases Research Citation Impact. IEEE Data Engineering Bulletin 28(4) pp. 39-47.Is the OAA, then, QB or QA (or both)? There is no way to determine this unless the causality is controlled by randomly imposing the self-archiving on a subset of a sufficiently large and representative random sample of articles of all ages (but especially newborn ones) and comparing the effect across time.