If the goal is to gather as many valid evaluations of the students' knowledge as possible, then the students should be made aware that there are multiple versio

If the goal is to gather as many valid evaluations of the students' knowledge as possible, then the students should be made aware that there are multiple versions of the tests. Students who might have tried to cheat if all the tests were identical would probably not attempt to cheat if they knew that there were multiple versions. By reducing the frequency of cheating you increase the frequency of valid evaluations of students' knowledge, and that should be the goal of testing I think. Making it easier for students to cheat just makes the tests more and more of an evaluation of the students' morals, not of their knowledge about whatever the test supposed to be about. TL;DR: Sometimes "opportunity makes the thief", so telling students that there are multiple versions should reduce the amount of cheating by making them aware of how difficult cheating would be.