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Turnitin does not determine whether a paper has or has not been plagiarized. Similarity Reports are simply tools to help instructors locate potential sources of plagiarism, or text which may have been incorrectly cited. Therefore, only instructors can deem what is a "good" or "bad" score, as interpretation of the data can only be made by the instructor.

http://turnitin.force.com/pkb/articles/en_US/Knowledge_Article/Similarity-Report-Interpreting-Results-Instructor/ (retrieved on 13 Apr 2016, no longer accessible as of 7 December 2018)

[The Similarity Indices] do not reflect Turnitin’s assessment of whether a paper has or has not been plagiarized. [Similarity Reports] are simply a tool to help an instructor find sources that contain text similar to submitted papers. The decision to deem any work plagiarized must be made carefully, and only after in depth examination of both the submitted paper and suspect sources in accordance with the standards of the class and institution where the paper was submitted.

http://turnitin.com/en_us/training/instructor-training/about-originalitycheck (retrieved on 18 Sep 2014, no longer accessible as of 7 December 2018)