Training Humans
dilbert.com
dilbert.com
Why?
If you consistently reward every instance of good behavior, then when you stop, the good behavior will tail off remarkably quickly. You are caught in the trap of having to reward every good instance.
On the other hand, suppose the trainee has got used to being rewarded at random for good behavior. Then you stop entirely.
The first time the trainee says "Yes? Oh, no. Oh well, maybe next time." The next time the trainee says "Yes? Oh, no. Oh well, maybe next time."
And on it goes. Randomly reinforced good behavior persists for a very long time after the rewards stop. Very occasional rewards makes it even longer.
This is also the most effective way to combat extinction. Near extinction, there is usually an extinction burst. And to keep the person or animal motivated, you want to up the rewards when they are about to quit. Just think of someone addicted to drugs or alcohol. When they are trying to quit, they get cravings, and can binge on the substance. http://en.wikipedia.org/wiki/Extinction_(psychology)
If you want to mess with your probands mind, at some point switch silently to a variable time schedule. ("Variable Time (VT) provides reinforcement at an average variable time since last reinforcement, regardless of whether the subject has responded or not." from the Wikipedia article)
By the way, for example computer games usually combine several reinforcement techniques.