“Plagiarism — the use of other people’s words or ideas without giving proper credit — is only one part of the general problem of cheating. Anecdotal evidence as well as a few studies suggest that student cheating is much more widespread than usually recognised. (Although exams are thought to prevent cheating more than essays, actually the rate of cheating on exams may be higher than for any other assessment mode.)
Most cheating is undetected. For every student caught plagiarising, it is almost certain that many more plagiarisers escape detection.
Elimination of plagiarism by detection and penalties is labour-intensive and ultimately impossible. One article recommends that, to detect plagiarism, each essay be read four times. But this only picks up copying from published sources; copying from other essays, or false authorship of essays, is seldom detectable or provable.”
Brian Martin. Plagiarism by university students: the problem and some proposals. Tertangala, 20 July – 3 August 1992, p. 20.