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As a guide a returned percentage of below 15% would probably indicate that plagiarism has not occurred. However, if the matching text is one continuous block this could still be considered plagiarism. A high percentage would probably be anything over 25% (Yellow, orange, or red). This will depend on the students work and the requirements of the report or essay
Sources and Relevant Links
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Retrieving Student Turnitin Assignments
Viewing Instructor Comments on a Turnitin Submitted Paper - Students