What is a good Similarity Report score?
First of all, there is no perfect Turnitin similarity score. No two papers are the same, and therefore, no two Similarity Reports will tell the same story. The Similarity Report is a powerful resource, solely created for human interpretation.
Saying this, having too high a similarity score, while not meaning a student has plagiarised, may mean a student is relying too much on direct quotes or secondary sources. As a guide, somewhere between 15-20% might be considered a good score to aim for. If your score is a lot higher than this it is important to look at your Similarity Report in detail not just the overall score.
Your Similarity Report will show you the breakdown of your percentage. You may have a high similarity score but it may be made up of small percentages from many sources which is just good academic writing. Similarly, you may have a relatively low score but if this is made up of just one or two sources this could be a problem. It is more important to examine the Similarity Report than to just look at the overall score.