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Scientific citation is providing detailed reference in a scientific publication, typically a paper or book, to previous published (or occasionally private) communications which have a bearing on the subject of the new publication. The purpose of citations in original work is to allow readers of the paper to refer to cited work to assist them in judging the new work, source background information vital for future development, and acknowledge the contributions of earlier workers. Citations in, say, a review paper bring together many sources, often recent, in one place.
To a considerable extent the quality of work, in the absence of other criteria, is judged on the number of citations received, adjusting for the volume of work in the relevant topic. While this is not necessarily a reliable measure, counting citations is trivially easy; judging the merit of complex work can be very difficult.
Previous work may be cited regarding experimental procedures, apparatus, goals, previous theoretical results upon which the new work builds, theses, and so on. Typically such citations establish the general framework of influences and the mindset of research, and especially as "part of what science" it is, and to help determine who conducts the peer review.
Disciplined citation of prior works in mathematics and science is known at least as far back as Euclid. Late in the first millennium, Islamic scholars developed their practice of isnad, or "backing", which established the validity of sayings of Muhammad in the hadith. The Asharite school of early Muslim philosophy extended this into fiqh or jurisprudence, while the Mutazilite school used the traditional methods and applied them to science.
In some form, then, achieving authority for new work by citing accepted authorities is a near-universal idea among the peoples of the Mediterranean, whose educated people were exposed to one or other of these practices well before the European Renaissance and the emergence of the formal scientific method.
In patent law the citation of previous works, or prior art, helps establish the uniqueness of the invention being described. The focus in this practice is to claim originality for commercial purposes, and so the author is motivated to avoid citing works that cast doubt on its originality. Thus this does not appear to be "scientific" citation. Inventors and lawyers have a legal obligation to cite all relevant art; not to do so risks invalidating the patent. The patent examiner is obliged to list all further prior art found in searches.
Modern scientists are sometimes judged by the number of times their work is cited by others—this is actually a key indicator of the relative importance of a work in science. Accordingly individual scientists are motivated to have their own work cited early and often and as widely as possible, but all other scientists are motivated to eliminate unnecessary citations so as not to devalue this means of judgment. A formal citation index tracks which referred and reviewed papers have referred which other such papers. Baruch Lev and other advocates of accounting reform consider the number of times a patent is cited to be a significant metric of its quality, and thus of innovation.
Two econometric studies of economists' salaries estimated that, controlling for age and number of articles published by an economist, on average doubling the number of citations increases salary by 7 to 14 percent.
- Shaping Written Knowledge: The Genre and Activity of the Experimental Article in Science by Charles Bazerman 
- Laboratory Life: The Construction of Scientific Facts by Bruno Latour and Steve Woolgar
- The Structure of Scientific Revolutions by Thomas Kuhn