Turnitin is a tool which highlights similarity.
It does not determine if a paper has been plagiarised - that is the prerogative of the assessor.
Ordinarily, Turnitin will highlight any similarity to other sources.
The assessor (instructor/supervisor) has to decide if the similar text has been quoted correctly and attributed accurately.
The context is important, and Turnitin does not account for context.
To reiterate, Turnitin merely detects similarity to other sources.
A high Similarity Index could be due to:
- the student having quoted many other works to a significant extent;
- a long bibliography/reference list in that the paper;
- many common phrases in a particular field which cannot be avoided; and/or
- other legitimate reasons.
Another common reason is that Turnitin is comparing a student's submission to his/her previous submission. This particular case is easy to resolve, just exclude the source(s).
Additional reading
Plagiarism.org article on How similar is too similar? (retrieved on 7 December 2018)
Turnitin blog post on Effective solutions for mastering the Similarity Report (retrieved on 7 December 2018)
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)