Have you ever wondered how many scholarly documents can be located via the public web? Two computer science researchers from the University of Pennsylvania set out to tackle this daunting task.
In a study recently published in PLOS ONE, Khasba and Giles (2014)  report that “at least 114 million English-language scholarly documents” can be found via the open web and approximately 27 million (or 24%) of these items are open access.
Their study provides an analysis, by discipline , of the “percentage of publicly available scholarly documents” included in Google Scholar. According to their research, the field of Computer Science is at the high end with approximately 50% of documents freely available via Google Scholar. While Engineering, Materials Science, and Agricultural Science were tied at the low end with each of these fields having 12% of documents freely available from Google Scholar.
Their research results were included in a recent discussion in the journal Nature, "Price of knowledge prompts reflection" (Woolston 2014). 
Khabsa, M. & Giles, C. L. 2014. The Number of Scholarly Documents on the Public Web. PLoS ONE 9, e93949.
Woolston, C. 2014. Price of knowledge prompts reflection. Nature 509: 537. Published online 28 May 2014.