Never make judgements about student submissions in terms of marks or absence/presence of plagiarism based on the overall similarity percentage. It is always about what the match consists of. There is no ideal overall similarity, nor any figure below which an assignment is ok, and above which it is not ok.
Turnitin has both:
- False positives (where matches are highlighted to text that is not problematic, eg cover sheets, institutional addresses, reference lists, commonly used references, templates, quotations, appropriately cited tables, standard academic phrases, etc.), and
- False negatives (where no matches are found, but markers may find similarity to texts by using other software or Google, or checking source texts themselves).
High scores do not necessarily indicate plagiarism; reasons could include lists of appendices or given tables, and extensive use of quotations.
Low scores do not necessarily indicate absence of plagiarism, as Turnitin does not find all plagiarism, and essay writing companies are known to produce texts with 0% match. Low scores may also indicate poor or little research or use of sources, so students should not set out with the aim of keeping Turnitin scores low - they should set out with the aim to write effectively with sources.
More important than the overall similarity are the highest ranked sources in the breakdown of matches. If there are high individual matches, this may indicate plagiarism, or if correctly cited and formatted, may indicate over-reliance on sources, although there may still be understandable reasons for high matches.
The matches do not necessarily show the sources the student used; they show the latest use of the words on the Turnitin database. Very often, this means the use of words from a published source by another student; small matches to many other students’ work just indicates that they are writing about the same subject using the same sources, it is not collusion.