Good exit interviews would likely be the best way to get at which factors are most prominent, and then from that data, you might be able to create a better context or set of variables that the computer question is part of. Same too with computers and writing.
Technology changes what writing is, how it's done, when, where, and why.
It changes how we think about writing and when we write and what writing is, and in a lot of classrooms these changes aren't noticed too well because students are asked to write primarily 4-7 page essays of the kind that used to be produced prior to computers being ubiquitous. Yet how these essays are conceived and imagined and invented and written, and the context of that writing on a word processor within a classroom dynamic that might include as well file sharing, online discussions, online feedback, WWW reading and newsgroup posting, and so on, changes what writing is. I think it was David Bartholomae who looked at bunch of essays, some written in networked classrooms, others written by hand then typed up or wordp-ed up. He noticed, primarily, differences in voice between the two, if I recall correctly. The chapter was at the end of a book Trent Batson co-did with someone.