  Lawrence (2001)found computer science articles that were openly accessible
(OA) on the Web were cited more. We replicated this in physics. We tested
1,307,038 articles published across 12 years (1992-2003) in 10 disciplines
(Biology, Psychology, Sociology, Health, Political Science, Economics,
Education, Law, Business, Management). A robot trawls the Web for full-texts
using reference metadata ISI citation data (signal detectability d'=2.45; bias
= 0.52). Percentage OA (relative to total OA + NOA) articles varies from 5%-16%
(depending on discipline, year and country) and is slowly climbing annually
(correlation r=.76, sample size N=12, probability p < 0.005). Comparing OA and
NOA articles in the same journal/year, OA articles have consistently more
citations, the advantage varying from 36%-172% by discipline and year.
Comparing articles within six citation ranges (0, 1, 2-3, 4-7, 8-15, 16+
citations), the annual percentage of OA articles is growing significantly
faster than NOA within every citation range (r > .90, N=12, p < .0005) and the
effect is greater with the more highly cited articles (r = .98, N=6, p < .005).
Causality cannot be determined from these data, but our prior finding of a
similar pattern in physics, where percent OA is much higher (and even
approaches 100% in some subfields), makes it unlikely that the OA citation
advantage is merely or mostly a self-selection bias (for making only one's
better articles OA). Further research will analyze the effect's timing, causal
components and relation to other variables.
