Graded classroom activities have a greater impact on student performance than voluntary / "participation-only" activities, especially when applied to high-risk/underachieving students. This seems to have interesting implications when considering motivational factors in classrooms:

• Intrinsic motivation may be insufficient when students are given a choice ("“I’m not being graded on this stuff and I’ve got an organic chemistry midterm tomorrow; I’m not going to bother.)
• Extrinsic motivation may be more effective for increasing attendance and performance.