I find that when students answer a question on a written test, that answer takes on a quality of being "right", even if it isn't. The longer the interval between taking the test and getting the graded paper, the more solidly that initial answer gels, until it is almost impossible to change. Which is a very good reasonn to return test papers ASAP. In one of my classes, which meets only once a week, I use my lunch hour to grade the papers so students can get the corrections immediately.
