Interpreting the Similarity Index

This article explains how an instructor can use the Turnitin Similarity Index Score to determine a paper's originality.

Turnitin's similarity index matches content from a student submission to material that appears on the web and in the Turnitin database.

It is not a plagiarism detector itself, but can be reviewed by an instructor to determine if there is an originality concern.

Turnitin Similarity Colour Coding

Turnitin will automatically review a student submission against the similarity index, providing a similarity index score. The score will be color-coded according to the percent of text in the paper that is found to be similar to other sources.

Not all text identified as similar is an originality concern. Instructor interpretation will determine if the highlighted material is a "false flag" or a genuine originality concern.

False Flags Originality Concerns

  • Quoted material (setting dependent)
  • ┬áImproperly formatted quoted material
  • Bibliographies
  • Formulas
  • Assignment instructions
  • Assignments in which the answers are intended to be the same across students (maths, etc)
  • Extremely common phrasing (According to [author name], ...)


  • Material copied from a website
  • Exact or similar responses from another student
  • Paraphrasing that is too similar to the original
  • Text that matches submissions to the Turnitin database

Example 1: Instructions + Similar Responses

Here, the similarity index recognizes the instructions as other students have submitted the same assignment. Additionally, the responses will be relatively similar from student to student. Another submission form like Blackboard assignments would be suitable for this assignment.

Example 2: Improperly Formatted Quoted Material

In this example, the quotation marks are facing the wrong direction, resulting in this quoted material flagged by Turnitin in the Similarity Index.

Example 3: Improper Paraphrasing

In this case, a student has insufficiently paraphrased material. This results in sporadic highlighting from the same source. An instructor can determine to what extent this is considered plagiarism or consider revision as an option.

Allowing for multiple submissions can help students see highlighted similar material and allow them to revise and resubmit.

Example 4: Plagiarism

This is an example in which Turnitin is useful for identifying plagiarism, as large sections of text are highlighted as similar to other sources. Further indicating plagiarism, the only non-highlighted sections of text are quoted material.

For more details on the use of Turnitin and the Similarity Index, for staff and students, visit Turnitin documentation available at: