The sources of prior art are usually divided into "patent literature" and "non-patent literature". Patent literature can be found in public databases of patent offices like the European Patent Office (EPO) or the United States Patent and Trademark Office (USPTO).
Non-patent literature (NPL) is defined as any "technical document that is neither a patent nor a patent application and that is submitted by a party — such as an applicant, an opponent, or a third party — or cited by an examiner during patent prosecution."
Part of the NPL are scientific and technical articles. Worldwide, the number of scientific and technical articles published each year is almost as high as the number of patent applications per year and nearly double the number of granted patents per year:
Data (patent applications):
Data (granted patents):
Data (scientific and technical articles):