May 9, 2023, by Helen Whitehead

Turnitin: what does the % similarity score really mean?

At the University of Nottingham, as elsewhere, student assignment papers are submitted to the Turnitin service. This produces a Similarity score and report but it doesn’t indicate plagiarism or a lack of academic integrity. What Turnitin does is check a student’s work against a database of previous submissions, web pages and journals. If there are instances where a student’s writing is similar to, or matches against, one of the sources in the database, this will be flagged and a percentage score given to quantify the proportion of the submission that matches text in the database.

The ideal is definitely not a 0% match. It is perfectly normal for an assignment to match against some of Turnitin’s database. Even if a student has used quotes and has referenced correctly, there will be instances where a match will be found. The scores are just a measure of the amount of text that is matched. It does not directly judge the student’s work practices. It’s always up to academics and midconduct officers to use the similarity score and report as one tool to make an informed judgement.

What’s an acceptable percentage score? This will depend on the type of assignment and the subject area. For example, a lab report on an experiment that every first year student does is likely to have a lot of the same vocabulary and phrasing. A creative writing piece, on the other hand, should have a very low similarity score.

The kinds of text that may be included in the score can include:

• Matched text that is cited and referenced correctly.
• References listed at the end of the essay.
• Common phrases over a certain length (which are especially likely to be matched in lab and practical reports).
• Any text that the student may have copied from a website.

A large score may indicate over-reliance on direct quotations that are properly cited, good adherence to scientific reporting conventions, or even plagiarism/collusion.

Aiming for a 0% or low score will never be the way to use Turnitin Similarity scores and reports. Using them as a tool, with awareness of how Turnitin works, will inform academic integrity discussions. They can also help students to understand academic writing and how to use quotes effectively.

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