This is sooooo cool. The actual statistical methods they are using are pretty crude, but that’s fine. What’s important is their focus on the important goal. It’s sort of like Bill James or Nate Silver: if you’re using good information, and you’re focused on good questions, then the fancy statistics can come later (or from others).
In most educational efforts I know of (including my own), very little is done to target assessments to improvements for individual students. I really like what they’re doing here and it reminds me how I want to figure out how to do something similar in my own teaching and course materials.