  This paper introduces AntiPlag, an advanced plagiarism detection tool
intended for use on student texts. It is capable of both hermetic detection
that scrutinizes only local collections of documents (other students' texts and
lecture materials, for example) and web plagiarism detection, in which the aim
is at identifying instances of plagiarism that have been sourced from the
Internet. The main feature of the system is the sampling-based web plagiarism
detection, a novel approach to plagiarism detection that is based on combining
web and hermetic search technologies. The system uses standard web search
engines to locate documents on the Internet that might have been used as
sources of plagiarism by the writer of a text. During this sampling phase, the
suspected sources are downloaded, converted to ASCII text and saved to the
local database so that they can be later processed by using the hermetic
detection methods. We evaluated the system by using a test set that contained
instances of verbatim copying as well as texts in which plagiarism was
concealed by minor editing, replacing words with synonyms and by paraphrasing.
We compared the results achieved by AntiPlag to an earlier evaluation study of
four web plagiarism detection systems, SafeAssignment, TurnitIn, EVE2 and
Plagiarism-Finder. AntiPlag performed better than any of these systems,
achieving the accuracy 95.8% over all the test items.
