It is important to realize that the Similarity Index is NOT a "plagiarism index" – there is no score that is inherently "good" or "bad". 0% does not necessarily mean that everything is OK with the student's paper and 75% does not necessarily mean that the student should flunk. You have to look at the report and decide: what is going on here?
The Turnitin originality report shows the paper's text highlighted with any text that matches sources found in the Turnitin databases containing vast amounts of web content, previously submitted papers, and subscription-based journals and publications.
It is up to the person looking at the matches to decide whether the writer's intent matters. Some people care about intent; others do not. Sometimes it matters; sometimes it doesn't.
Since plagiarism is one of those topics that gets people all riled up (like politics and religion), there is no shortage of discussion on what it is, why it is complicated, and what to do about it. This is an important conversation with lots of shades of distinction – and that conversation should continue as today's "digital natives" have become the "new normal" and they have a very different way of relating to content.
So does Turnitin detect plagiarism? No - Turnitin offers a tool that helps educators (and their students) make informed evaluations of student work rapidly and move on to the important task of discerning what their students need in the way of instruction, correction or judicial action.
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