I’ve known fabulous programmers flame out in the quizzing cage and terrible ones excel. So unless you’re specifically hiring someone to design you the next sorting algorithm, making them do so on the white board is a poor gauge of future success.
The only reliable gauge I’ve found for future programmer success is looking at real code they’ve written, talking through bigger picture issues, and, if all that is swell, trying them out for size.
(1) In other words, the problem with using tests to ascertain a candidate’s capabilities is that tests don’t replicate a real work context.
(2) Here’s an alternative approach: How to run a job interview.
(3) See also The best question to ask in a job interview: Sonya Meloff and Jamie Scarborough.