I have found that if I add a question to a test that has the designation "EXTRA CREDIT", the students that do not really need the extra points will attempt the question. Those students who could use the extra points look at the question and feel "...if it is extra credit it must be extra hard. Too hard for me" and they skip the question. My solution is to simply put one or two extra questions on a test and calculate the percentage score not using those questions in the base line score. Example...52 questions on a test with each one being worth 2 points. This translates to a perfect paper (52 correct) scoring 104%. This way all the student try every question. PS. I teach math and this is also a learning sitution for the class. They get to calculate their own percentage grade.