Am I obliged to tell students that there are different versions of the exam? This past term, I was teaching in a collaborative classroom where students sit together at round tables. This works well for in-class activities and peer learning, but is the not exactly the best configuration on the day of a midterm or final. 

Concerned that some students might have wandering eyes – either inadvertently, or perhaps even intentionally – I came up with something that might help. Several of the exam questions required some basic arithmetic, so I made three different copies of the exam, where the numbers in the word problems varied a little bit. (Each version's problem was solved the same way, but the end result would be different.) 

Later, I was talking with my daughter about what I had done. She asked me if I told the students that not everyone was getting the exact same test. (I had not.) She told me about some classes she had taken where the instructor had done this same thing, but announced it ahead of time (saying something like, "There's no sense trying to copy an answer from a neighbor, because the problems vary.") She also said that one instructor had gone so far as to print the exams on different colored sheets of paper. (My daughter assumed that all the orange tests were the same, all the blue tests were the same, etc., but that was just conjecture.)

If this makes any difference, there was already one cheating incident (or a class project) earlier in the term. 

Also, I realize that it's often the "Show your work" part of an exam question that is the most important part, but that's not the case for this particular course. It's an introductory programming course, and I give small snippets of code, asking, "What does the output of this program look like?" So, for some of these questions, it would be very easy to copy an answer from another student's exam. 

This made me wonder: 


Am I under any obligation to share with my students the fact that there are multiple versions of the exam? Or is it okay to remain silent about the issue? 
Is there any good reason for doing it one way or the other?
Making different versions of the exam is standard practice in a lot universities when physical barriers against cheating are not available, when students take the test in different moments or with take away exams or homework.

Provided that difference of versions is not noticeable at first sight, not telling the students in advance won't harm honest students, but it will make dishonest students to unsuccessfully cheat in a way that will be easily noticeable - probably some students will produce the answer to their neighbours'questions. Therefore, I don't see an ethical problem in not telling them.

In the end it depends on what is your goal: If you want to catch dishonest students cheating, don't tell them, but if you prefer discouraging dishonest students from trying to cheat, tell them. When faced with that dilemma I usually choose the later.

Of course, by telling them you will give cheaters some useful information on how to cheat, but that should be addressed in another way.