  In a study of articles published in the Proceedings of the National Academy
of Sciences, Gunther Eysenbach discovered a significant citation advantage for
those articles made freely-available upon publication (Eysenbach 2006). While
the author attempted to control for confounding factors that may have explained
the citation differential, the study was unable to control for characteristics
of the article that may have led some authors to pay the additional page
charges ($1,000) for immediate OA status. OA articles published in PNAS were
more than twice as likely to be featured on the front cover of the journal
(3.3% vs. 1.4%), nearly twice as likely to be picked up by the media (15% vs.
8%) and when cited reached, on average, nearly twice as many news outlets as
subscription-based articles (4.2 vs. 2.6). The citation advantage of Open
Access articles in PNAS may likely be explained by a quality differential and
the amplification of media effects.
