  Most refereed journal articles today are published in subscription journals,
accessible only to subscribing institutions, hence losing considerable research
impact. Making articles freely accessible online ("Open Access," OA) maximizes
their impact. Articles can be made OA in two ways: by self-archiving them on
the web ("Green OA") or by publishing them in OA journals ("Gold OA"). We
compared the percent and growth rate of Green and Gold OA for 14 disciplines in
two random samples of 1300 articles per discipline out of the 12,500 journals
indexed by Thomson-Reuters-ISI using a robot that trawled the web for OA
full-texts. We sampled in 2009 and 2011 for publication year ranges 1998-2006
and 2005-2010, respectively. Green OA (21.4%) exceeds Gold OA (2.4%) in
proportion and growth rate in all but the biomedical disciplines, probably
because it can be provided for all journals articles and does not require
paying extra Gold OA publication fees. The spontaneous overall OA growth rate
is still very slow (about 1% per year). If institutions make Green OA
self-archiving mandatory, however, it triples percent Green OA as well as
accelerating its growth rate.
