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The best, activity-based model I know of by Bjork

We used the databases of ISI and Ulrich's as our primary sources and estimate that the total number of articles published in 2006 by 23 750 journals was approximately 1 350 000.\nUsing this number as denominator it was also possible to estimate the number of articles which are openly available on the web in primary OA journals (gold OA). This share turned out to be 4.6 % for the year 2006. In addition at least a further 3.5 % was available after an embargo period of usually one year, bringing the total share of gold OA to 8.1%\nUsing a random sample of articles, we also tried to estimate the proportion of the articles published which are available as copies deposited in e-print repositories or homepages (green OA). Based on the article title a web search engine was used to search for a freely downloadable full-text version. For 11.3 % a usable copy was found. Combining these two figures we estimate that 19.4 % of the total yearly output can be accessed freely.