Of the students I studied, procrastinators were half as likely as non-procrastinators to report proofreading. At first glace this seems like a useful statistic, but I'm not entirely sure.
I didn't set up my questionnaire appropriately to actually study this question. I did not ask students whether they had proofread, merely what changes they had made to their papers. It's possible that students proofread but neglected to mention it. For instance, an instructor might have made a point in class about how proofreading could not stand in for more substantial revision. So when asked what changes they made to their papers, students might have felt they should report the changes their instructor liked to hear about, like organization or elaboration.
In order to determine whether a student had proofread their paper, I simply read what changes they reported making to their paper over time. If they said something along the lines of "fixed grammar" or "changed spelling," I counted that as proofreading. One student said he "added some commas," which I decided fell into the category of proofreading. I did not count "sentence structure" as proofreading.
Studying whether procrastinators proofread would properly require another study, preferably one that validated the reports with direct evidence of the students' writing processes. Since I'm only looking at the end product, the amount of surface errors more appropriate to measure. It just doesn't tell as much about the writing process, if that's what you're interested in.
Another concern I have is that students in Matt's class, procrastinators or not, were more than seven times more likely to report proofreading than those in Lee's class. So while procrastination does appear to correlate to proofreading, it isn't the best predictor of my data.