When does excessive collaboration become plagiarism?
I'm presenting this as an answer to preserve it. It isn't a direct response to the question, I know. 

Let me draw out a conclusion from @BenI.'s answer. 

The plagiarism "problem" in education seems to flow from a (IMO) misguided notion that all student intellectual growth must flow from that individual only without any collaboration whatever. I used to believe that myself, especially when teaching math, but less so in CS. 

Certainly work in isolation can lead to A-Ha moments that indicate growth, but can also lead to deep frustration and abandonment. But collaborative work while it may lessen the former somewhat certainly tends to eliminate the latter. Many years ago, when I was a math student, most academic papers were by individuals. That is much less so now, and in CS, collaborative work is the norm. 

Try to teach and organize your course so that plagiarism isn't an issue. Encourage peer work, peer teaching, peer evaluation, and all the rest. Find a way to evaluate individuals without requiring that their work be done in isolation. It better matches the work-a-day world that many of them will enter eventually. It reduces headaches, guilt, punishment. It encourages learning, sharing, and growth. Be a shepherd, not a judge.

How you do it is a question of course organization and project selection. Permitting collaboration also allows you to utilize larger, more challenging, projects than you might be comfortable with otherwise. You can impose rules in programming courses, of course, requiring role changes while pair-programming (driver &lt;-> navigator) for example. You can sometimes choose teams or pairs yourself, and sometimes let them choose. For larger teams you can even use sandlot-baseball team choosing, which I've found to work. (You choose captains, the captains alternately choose additional members until everyone is on a team).

Don't just permit them to work together. Try to find a way to require them to do it. It is harder for off-site assignments, but not always impossible, especially if your institution provides some sort of common workspace for students. 



The cited research seems to be about improving teaching, not about detecting plagiarism. No student should be accused of plagiarism based on statistical measures only. The paper seems to correctly make that point.