I can’t give specific answers to your questions, since writing isn’t part of my research interests, but this discussion posted on ResearchGate might be of interest to you:
In case you’re not familiar with this academic network, membership is free, and the network is quite active.
I hope this helps!
Like Dr. Cruz-Ferreira, I am not a specialist in (research about) writing, so I won't give you direct answers to the questions you're posing.
You don't say where you are, but if you're near a research library for your graduate program, I'd go ask the librarians with help using an abstracting journal (like Linguistics and Language Behavior), which gives brief abstracts for articles from 1200 or more journals in each issue. This will help you determine whether others have done research on this topic, and how they measured change.
Your instructor in the course is likely more familiar with this area of research and would be the best at guiding you to find a research question that's sufficient for the course, and for which metrics already have been tried with some success.
Nancy J. Frishberg