I have a homework assignment where I require students to create a set of pivot charts in Microsoft Excel. I know that I can reduce cheating by changing datasets or chart specifications around, but because of the complexity of the assignments and my practical constraints in not being able to mark more than five or so different versions, this does not prevent students from finding those who have the same specifications as themselves and copying among themselves. (I am coordinating a course with over 700 students, so I can only do so much as far as alternate versions go. The students are well connected to each other by Facebook, so communicating among each is quite feasible for those who want to cheat.)
On one hand, in my situation, creating alternate versions is difficult because of the complexity of the assignment, and its effectiveness is limited because hard-working cheaters can cheat anyways. On the other hand, I believe that if I can effectively detect cheating after the fact, then giving a strong warning to students about this could serve as an effective deterrent that should greatly reduce the incidence of cheating.
Are there features in Microsoft Excel that make it easy to detect cheating by copying between students? I don't expect to easily detect if a student copies elements like pivot tables from one file to another, but perhaps at least I might be able to detect if a student copied someone else's file altogether. I'm not necessarily looking for anything foolproof--I expect that the cheaters in this assignment would be those who don't even know basic things like how to manually change author information in an Excel file.
Speaking of which, unfortunately, as far as I can tell, Excel overwrites author information with the last person to save the file, so I couldn't even use that to tell if someone has copied a file created by someone else.
I would appreciate any suggestions.
Some answers have suggested asking students to submit a written explanation of their reasoning. Although I mentioned that there are over 700 students, let me be more explicit: I do not consider solutions that require reading and marking additional textual submissions to be feasible for my context. On one hand, there are multiple instructors, so any instructor would only have to mark a few dozen assignments. On the other hand, if there is cheating across sections, then reading text submissions is not a feasible verification technique. Thus, I was looking for a solution that could easily indicate that a student's submission contained undeniable traces of some other student's work.