Symptom

Student submits paper once, revises, then submits again. Similarity Index of the revised paper is higher than the original paper.

Cause

Most likely, Turnitin is comparing the revised paper to a previous submission.

This happens because:

1. the instructor:

   1. did not change the Optional Settings to Generate Similarity Reports for student submissions... immediately (can overwrite until due date), and/or

   2. required the students to submit drafts to earlier Turnitin submission folder(s) and did not change the Optional Settings to Submissions to this assignment will be stored in...do not store the submitted papers.
      
      

2. students are required to submit reports at various stages as a group, but a different person submits each time, resulting in one submission being compared to the other.
   
   

3. student submits to the wrong course (class) and/or wrong assignment folder.
   
   

4. (in Quick Submit - staff only) the staff checked the Add file to Turnitin's repository option.

   1. Once a file is added to Turnitin's repository, the submission of a revised paper is checked against the earlier submission.

Solution

Staff can exclude the source(s) leading to the skewed Similarity Index.

Also see Why is a student paper self matching? (retrieved 21 April 2023)