Some searchers found Google Scholar to be of comparable quality and utility to subscription-based databases when looking at citations of articles in some specific journals. The reviews recognize that its "cited by" feature in particular poses serious competition to Scopus and Web of Science. A study looking at the biomedical field found citation information in Google Scholar to be "sometimes inadequate, and less often updated". The coverage of Google Scholar may vary by discipline compared to other general databases. Google Scholar strives to include as many journals as possible, including predatory journals, which may lack academic rigor. Specialists on predatory journals say that these kinds of journals "have polluted the global scientific record with pseudo-science" and "that Google Scholar dutifully and perhaps blindly includes in its central index." Google Scholar does not publish a list of journals crawled or publishers included, and the frequency of its updates is uncertain. Bibliometric evidence suggests Google Scholar's coverage of the sciences and social sciences is competitive with other academic databases; as of 2017, Scholar's coverage of the arts and humanities has not been investigated empirically and Scholar's utility for disciplines in these fields remains ambiguous. Especially early on, some publishers did not allow Scholar to crawl their journals. Elsevier journals have been included since mid-2007, when Elsevier began to make most of its ScienceDirect content available to Google Scholar and Google's web search. However, a 2014 study estimates that Google Scholar can find almost 90% (approximately 100 million) of all scholarly documents on the Web written in English. Large-scale longitudinal studies have found between 40 and 60 percent of scientific articles are available in full text via Google Scholar links. Google Scholar puts high weight on citation counts in its ranking algorithm and therefore is being criticized for strengthening the Matthew effect; as highly cited papers appear in top positions they gain more citations while new papers hardly appear in top positions and therefore get less attention by the users of Google Scholar and hence fewer citations. Google Scholar effect is a phenomenon when some researchers pick and cite works appearing in the top results on Google Scholar regardless of their contribution to the citing publication because they automatically assume these works' credibility and believe that editors, reviewers, and readers expect to see these citations. Google Scholar has problems identifying publications on the arXiv preprint server correctly. Interpunctuation characters in titles produce wrong