There is no official statement about how big the Scholar search index is, but unofficial estimates are in the range of about 160 millions, and it is supposed to continue to grow by several millions each year. Yet, Google Scholar does not return all resources that you may get in search at you local library catalog. For example, a library database could return podcasts, videos, articles, statistics, or special collections. For now, Google Scholar has only the following publication types:
- Journal articles: articles published in journals. It's a mixture of articles from peer reviewed journals, predatory journals and pre-print archives.
- Books: Links to the Google limited version of the text, when possible.
- Book chapters: Chapters within a book, sometimes they are also electronically available.
- Book reviews: Reviews of books, but it is not always apparent that it is a review from the search result.
- Conference proceedings:- Papers written as part of a conference, typically used as part of presentation at the conference.
- Court opinions
- Patents: Google Scholar only searches patents if the option is selected in the search settings described above.
The information in Google Scholar is not cataloged by professionals. The quality of metadata will depend heavily on the source that Google Scholar is pulling the information from. This is a much different process to how information is collected and indexed in scholarly databases such as Scopus or Web of Science.