I'm not requiring them to be logged in though.... Couldn't I look in the database and see if there is a hash there - which would mean they took the survey, no hash means they didn't? I'm not saying I would. But, the ethics committee would say that if I have access, someone else could gain access and potentially compromise the data.
See, I'm not even allowed to know that kind of information.
[This is all built on the fact that I'm saying it's a completely anonymous survey.]