Google Scholar is a freely accessible web search engine that indexes the full text or metadata of scholarly literature across an array of publishing formats and disciplines. Released in beta in November 2004, the Google Scholar index includes peer-reviewed online academic journals and books, conference papers, theses and dissertations, preprints, abstracts, technical reports, and other scholarly literature, including court opinions and patents. Google Scholar uses a web crawler, or web robot, to identify files for inclusion in the search results. For content to be indexed in Google Scholar, it must meet certain specified criteria. An earlier statistical estimate published in PLOS One using a mark and recapture method estimated approximately 80–90% coverage of all articles published in English with an estimate of 100 million. This estimate also determined how many documents were freely available on the internet. Google Scholar has been criticized for not vetting journals and for including predatory journals in its index. The University of Michigan Library and other libraries whose collections Google scanned for Google Books and Google Scholar retained copies of the scans and have used them to create the HathiTrust Digital Library. History Google Scholar arose out of a discussion between Alex Verstak and Anurag Acharya, both of whom were then working on building Google's main web index. Their goal was to "make the world's problem solvers 10% more efficient" by allowing easier and more accurate access to scientific knowledge. This goal is reflected in the Google Scholar's advertising slogan "Stand on the shoulders of giants", which was taken from an idea attributed to Bernard of Chartres, quoted by Isaac Newton, and is a nod to the scholars who have contributed to their fields over the centuries, providing the foundation for new intellectual achievements. One of the original sources for the texts in Google Scholar is the University of Michigan's print collection. Scholar has gained a range of features over time. In 2006, a citation importing feature was implemented supporting bibliography managers, such as RefWorks, RefMan, EndNote, and BibTeX. In 2007, Acharya announced that Google Scholar had started a program to digitize and host journal articles in agreement with their publishers, an effort separate from Google Books, whose scans of older journals do not include the metadata required for identifying specific articles in specific issues. In 2011, Google removed Scholar from the toolbars on its search pages, making it both less easily accessible and less