I'm extremely sure that we do get false positives/negatives. Nevertheless I believe the external factors swing toward it not mattering an enormous amount. I'm more than willing to be proved wrong. As I said, we do very carefully assess relevant skills; it just doesn't seem to make a lot of difference to the end decision. This can obviously be attributed completely to bias, to retroactively reassessing, but I don't think that's correct. The crux is a. can I get on with this person, b. can I sit beside this person for 8 hours a day, c. do they want to learn/can they teach us. The technical side is less important than these things, and is easier to filter in many respects.