[ZeroGravitas]

A couple of older posts about what they're doing:

How Khan Academy is using Machine Learning to Assess Student Mastery

http://david-hu.com/2011/11/02/how-khan-academy-is-using-mac...

Khan Academy: Machine Learning → Measurable Learning

http://derandomized.com/post/51729670543/khan-academy-machin...

I think the latest Dashboard design that they've just (soft-?) launched is based on this work. When you first start it asks you about 10 questions and based on your performance on that short "Maths pre-test" guesses what you know and what you don't across the whole of Maths.