I felt that three exams were not sufficient. I decreased the value of each quiz and in some cases increased the number of questions. Quiz questions are now worth 0.3-0.5 pts each, but with the automatic grading in WebCT, this is no problem. For the first three quizzes, I used a feature of the new version of WebCT that allows quizzes to be delivered one question at a time, with no returning to previous questions. However, students seemed to find this stressful, so I went to a compromise.
Questions appear one at a time, but can be revisited.
I also gave a generous but enforced time limit for each quiz, at least twice the amount of time the slowest student might require. Quizzes are open book, so I did not want students to feel overly pressured. In the fall, students basically had unlimited time. We are now about half way through the term, so I will have more to report at the end. I am curious to find out if this class is as positive as their counterparts in the fall.