do you screen them over the phone? seems to me like it would be much easier and less depressing if you just gave them the work, gave them an hour, and told them to email the results back to you. stuff that is nontrivially difficult such that they can't just search and find answers. they have to know.

edit: i think i'd prefer having to do this myself, as well, because i'm a shy person in person and i find myself floundering when i'm on the phone and have to "explain my thought process" to someone as i'm working through it. i'd much rather write down an explanation.