Continuum from collaboration on homework to copying Some of the work I grade does not seem to be copied verbatim, but also seems close enough that one student or another is being a parrot and does not totally understood what they are saying. My students are working under a collaboration policy similar to the example on this page, which says "Peer collaboration is highly encouraged. It is a highly efficient and fun way to learn [statistics]. However, your homework must be entirely your own work and in your own words." If Alice and Bob discuss problem 3, shouldn't I expect Alice's words, mathematical notation, and problem-solving strategies to appear in Bob's writeup? Please advise me on how to reconcile "collaboration is great etc." with "homework must be entirely your own work."

I have browsed all the other questions tagged grading and a few besides; none seemed relevant.
That is why I have a rule that for each homework a group of students is selected "at random", and asked by the TA to explain what they turned in. The grade of the interrogation replaces the homework grade.

Rationale is that I really don't care (too much) if they copied from the Internet, got it in an obscure book somewhere, or worked it out in a group. I want them to understand, and this forces them to at least be somewhat familiar with the solution they hand in.