Hi Michael,
Good approach and good way to gain insight into where your students are coming from. I use the index card as well. A side note I have found in using a 3x5 card is that my students like the small card because that way they don't have to write much. They are all a bit worried at the beginning of the class about their abilities, what I want from them, etc. The cards let them share a little bit of something they know for sure, themselves.
Gary
