Previous versions of DRILL offered only one exam, covering the topic of common algebra errors made by calculus students. It has been offered to students in firstsemester calculus four times, dating back to 1998. Students were required to take the untimed exam at least once, and a small incentive was provided for successful completion. Students were free to take it wherever, whenever, and as many times as they wanted. Some students took advantage of this freedom to take the exam hundreds of times. Generally all examrelated activity took place within the first two weeks of the course.
A total of 145 students have taken that exam at least once, of which 68 completed it successfully. The others did not manage to receive a passing score, which was an unforgiving 100%. We distinguish here two groups of students  those who went on to earn less than B in the entire course (moderate to poor calculus performance), and those students who went on to earn less than C (poor calculus performance). Naturally, we want to decrease the size of each of these groups, which would correspond to more students doing well. The following table shows aggregate data  details are available upon request.
% of students who passed the exam 
% of students who did not pass the exam 

Earned < B  43  68 
Earned < C  19  44 
We can offer two explanations for the dramatic semesterlong differences between groups that did or did not complete a single exam. Likely, both are true to some degree.
DRILL still has a limited set of question models  certain combinations of skills and objects are not fully implemented. By making additional question models, it could be useful in other courses as well.