Turnitin does not check for plagiarism in a piece of work. Instead, it checks a student's work against the Turnitin  database, and if there are instances where a student's writing is similar to, or matches against, one of the Turnitin  sources, it will flag this for you to review. The Turnitin database includes billions of web pages: both current and archived content from the internet, a repository of works students have submitted to Turnitin in the past, and a collection of documents, which comprises thousands of periodicals, journals, and publications.

It is perfectly natural for an assignment to match against some of the items in the Turnitin database. If a student has used quotes and has referenced correctly, there will be instances where Turnitin will find a match. The similarity score simply highlights of any problem areas in a student's paper; you can then use this as an investigative tool, in order to determine if the match is or is not acceptable.

Similarity Score

Similarity Reports provide a summary of matching or highly similar text found in a submitted paper. When a Similarity Report is available for viewing, a similarity score percentage will be made available. Similarity Reports that have not yet finished generating are represented by a grayed out icon in the Similarity column. Reports that are not available may not have generated yet, or assignment settings may be delaying the generation of the report.

Note: Overwritten or resubmitted papers may not generate a new Similarity Report for a full 24 hours. This delay is automatic and allows resubmissions to correctly generate without matching to the previous draft.

The color of the report icon indicates the similarity score of the paper, based on the amount of matching or similar text that was uncovered. The percentage range is 0% to 100%. The possible similarity ranges are:

Refining a Similarity Report

To refine your students' similarity scores, consider the following Similarity Report filters:

Exclude small sources (measured by word number or percentage)

You can exclude sources in the source list that are below the threshold set by you. For example, if the threshold is set at 3%, any 1% or 2% match would be removed from the current report mode's source list (Match Overview or All Sources).

Quotes and bibliography items are widely reused and generally fail to demonstrate original writing. You may wish to exclude quotes, bibliographies, and items of a similar nature from influencing your students' similarity scores.

Exclude full sources

The source exclusion feature is generally used when a paper has been submitted more than once to Turnitin (possibly as draft submissions). Sources may also be excluded when an instructor agrees that students use a certain source for their writing.

If you believe an item may have been added to the Turnitin database since a Similarity Report was last generated (this could be a website, journal article, or even another student’s paper), you can generate a new report to receive an up-to-date score.