I my classroom there is virtually no extra credit. The biggest reason, it distorts grades. If a student is proficient in 80% of the material being tested then they should get an 80%. If they can do anything (other than become more proficient at the material) for extra credit, your are artificially inflating their grades. As Jonas mentioned it is possible to have extra credit that will improve students skills or knowledge in an area, but unless they are re-tested in the material I don’t think it should improve their scores.
To help out those situations were a student has a bad day I have a test re-take policy in all my classes. If you do not like your test grade you can come talk to me and I will help you come up with a plan for re-testing. This plan various greatly, but in general we work out a plan about which material you are going to study, when your going to do it, and how your going to study the material. I try to keep student accountable to their plan of action and then they do a re-take. (Yes it is a pain to make other tests) Although it is time consuming if you don’t allow students to actually learn something they don’t know well, aren’t you sending the message that the material isn’t important. You didn’t learn this material but we have other things to learn now, too bad for you.
Many major tests allow you to do retakes for full credit. MCAT (intro to med. school), SAT, auto mechanics license, pilots licences, PRAXIS (teacher certification) bar exams, CPA licences, drivers license. If you don’t allow retakes your aren’t teaching so that the student learns the material, your are teaching so that the student learns it on the conveyor belt. There is no reason a child has to learn fraction during one week in October and then all learning of fractions must subserviate to other instructional material. This doesn’t reflect real life.