Dear Eddy,

At a slightly higher level I would recommend looking at:

Lavigne, N. C., Salkind, S. J. and Yan, J. (2008) Exploring college students’ mental representations of inferential statistics. Journal of Mathematical Behavior, 27(1), pp. 11-32.

Perhaps their concept mapping plus interview technique could be applied in your context as well? I like the way they represent changes in understanding through dynamic concept maps.