I disagree that a whiteboard session can be considered a work sample test. Whats wrong with 'complete steps 1,2 & 3 and i'll be back in 30 minutes'?
For engineering we actually found giving a 2-3 hr 1 problem test has been a better predictor, and 3-4 30 minute face to face (usually 2 on skype prior to onsite Test, then 2 follow ups). Alone time gives the candidate time to digest and solve the problem on their own which is more 'real world' like and shows what can be done on their own. Some questions are intentionally vague to compensate for 'able to make decisions with little information'. We have no right answer but will ask why you made these decisions. This frees up our staff time tremendously and weeds out those that may need a lot of hand holding early on.