I like to ask true-false questions on exams, because I feel that they can be an efficient way to assess students' understanding of concepts and ability to apply them to somewhat unfamiliar situations. In general, I'm very happy with true-false questions, but there is one annoyance that I have never figured out how to deal with correctly. Namely, should students be asked to justify or explain their answers to true-false questions?
Some reasons to ask for explanations:
- It may inspire them to think harder.
- It makes it possible to give partial credit.
- It reduces the incentive to guess randomly.
- It's easy to give the right answer for a wrong reason, and if I don't ask for an explanation I may never notice. (I find this reason the most compelling.)
Some reasons not to ask for explanations:
- Writing an explanation takes a lot more time than just writing "true" or "false", partially negating the efficiency of true-false questions versus "free-response" questions. In particular, a student who has no clue may spend a long time writing rubbish in hopes of partial credit.
- For some otherwise nice true-false questions, it's not clear what sort of "explanation" could be given. Sometimes I find that one of "true" and "false" (often the correct one) has a much easier explanation than the other. It's also hard to communicate to the students what level of detail and precision is expected, which makes it hard to grade fairly.
- Often students will give an explanation that's basically correct, but also contains inaccuracies or false statements. If I let it slide, then they won't learn to be careful with language; but it's hard to take off something while still giving credit for basic correctness when I usually assign only a couple of points per true-false question. As with #1, this pushes the true-false question towards more of a free-response question.
- Not a really good reason, but for completeness, I'll include it: it takes longer to grade.
Is there a good solution? Do you ask your students to give explanations for true-false questions? How do you deal with the issues raised above?