It is relatively easy to compare the students’ matrices with you master copy. You can scan them one by one, indicating incorrect responses on each student’s matrix with a slash or an X and, at the same time, keeping a running tally of incorrect responses on a larger copy of the matrix with empty cells. (By investing a little more preparation time, you can create an answer format that can be scanned electronically.) Or you can simply count all the pluses and minuses for each cell and tally them on an oversized copy. Look for those cells where several students made the wrong choice and see whether there are patterns in the errors. Are students paying more attention to certain features than to others? Are they failing to notice defining differences of specific kinds that would be obvious to an expert?