Google Scholar is a public Internet search engine which indexes the full text or metadata of scientific literature in various formats and publishing disciplines. Released in beta in November 2004. The Google Scholar Index covers the majority of peer-reviewed online journals and books, conference papers, thesis and dissertations, preprints, summaries, technical reports and other scientific literature, including court opinions and patents . Although Google does not publish the size of the Google Scholar database, researchers have estimated that it contains around 389 million documents, including articles, quotes and patents, making it the world's largest academic search engine in January 2018. Previously, the size was estimated at 160 million documents for May 2014. Earlier statistical estimates published in PLOS ONE using the Mark and recapture method estimated approximately 80-90% of the coverage of all articles published in English with an estimated 100 million. This estimate also determined how many documents were freely available online.