The University of Portsmouth uses the Turnitin service to provide facilities for plagiarism detection, online marking and as a development tool for academic writing, although most users are interesting in one thing – a number.
Contained within the Originality Report is a Similarity Score out of 100, which many users wrongly believe to a be plagiarism score with a magic number, at which in can be conclusively determined whether plagiarism has or has not occurred. The problem is, this figure can be manipulated, there will also be mitigating circumstances and lastly let us not forget the system is not perfect either – there will be some margin for error.
Crudely speaking the Similarity Score number is a percentage of the words in your document which matched text from other documents that Turnitin searched against. For shorter assignments with a direct question and consequently a more concise correct answer may well therefore see higher score when compared to a longer assignment with more scope to include to include diverse material.
The number of students in your class and whether the assignment has been set in previous years (or at different institutions) may limit the scope for truly original material, that’s not to say a very high score is necessarily acceptable however it does mean that the latest content may not be unique for genuine reasons. An assignment based upon group work is also a recipe for a higher than usual Similarity Score since students are likely to be working from the same research, data and figures so will in all likelihood draw the same conclusions.
What does Turnitin check an assignment against? There are stored student papers in both a global central repository and the University of Portsmouth own repository (where we might store more sensitive documents). Turnitin also searches against material found on the internet and can check journals, periodicals and publications. Personally I would check against everything, if the service is available, use it.
Turnitin offers several filters which may be toggled, for example whether to include or exclude bibliographic references. Personally I cannot think of a reason why you want to include bibliographic references in the Similarity Score as citing sources is a requirement of good academic writing. That said if the assignment were a lab report and references were not expected then it might be safer to include bibliographic references just in case the Turnitin software incorrectly identified a bibliography and consequently excluded all of the text that followed. You can also toggle quoted material, quotes would not normally be considered within a plagiarism report although the volume of them may indicate a lack of original content from the author. Where quoted material is excluded from the Originality Report, Turnitin helpfully points out when more than 15% of the paper is quoted material. The final filter is for small matches, usually matches of 3-4 words are rather inconsequential, you may also have longer phrases that appear repeatedly throughout the assignment – you can exclude this from being repeatedly matched and skewing the Similarity Score using the ‘exclude small matches’ filter. Personally I use all the filters, excluding bibliographic references, quoted material and small matches – I can always turn them back on later when reviewing a paper if I am suspicious.
So after searching against all of the available material, excluding bibliographic references, quoted material and small matches, what is the magic number? Well, the magic number is… the number at which you become suspicious of course!
Finally, to wrap up this post, and just in case a concerned student has stumbled across this blog post, I would like to emphasise that if they know they have not deliberately plagiarised then they have nothing to worry about. If they are concerned that they have used another source and may not have referenced it properly, then guidance is available from the Academic Skills Unit (https://kb.myport.ac.uk/Article/Index/12/4?id=2747)
Telephone: +44 (0)23 9284 3462
Or, visit the Academic Skills Unit in person during our opening hours:
Third floor Reception, The Nuffield Centre
St Michael’s Road