Paul,
A challenge for any instructor is to use a grading system that accurately measures learning. Although I believe it's possible for an extra credit assignment to add to student learning, I also think that it usually distorts the grading system by virtue of it being "extra". So if points and grades are the focus, EC is OK to use. If we want to measure learning, we have to rely on the grading system in place at the beginning of the course.
Barry Westling
