How to prevent cheating on take-home exams For certain undergrad classes, I assign a take-home exam, rather than the more familiar in-class exam. When I distribute the exam, I remind the students that this is an individual exam, and that they may not work in groups, and then I add "Believe me, I can tell when you cheat". This, however, is a bluff. Unless it is superobvious, I can't tell if students work together; I only tell them I can to scare them into honoring the rules. 

Are there any ways, other than lying to the students, to prevent (or at least minimize) this type of cheating in take-home exams?
One way to do it: if the assignment has many small questions, you can make it more difficult taking a random draw from a bigger pool for each student, so they are all slightly different. So, any pair of students would have just a portion of them in common. This can appear unfair, but it should even out if you do it many times.

But, if this were a fight, you would be on the losing side; for any strategy you can come up with, someone else would find the way to hack it. You should instead focus on making people not want to cheat.


Make the problems interesting challenges, not mechanical tasks. If it involves some creative thinking it is less likely that two students arrive independently to the same solution (and even less to arrive to the same solutions in each exercise).
But make them approachable. If they look impossible, it is more tempting to cheat. In a course I took recently, we had to solve an easier version of a problem, and apply it to a more difficult one. We only had to hand in the difficult version, but handing in only the easy part gave also points.


For what I have seen from a student perspective, the more advanced the course, the less likely cheating is, and the more frowned upon by the other students is.