Thus, we start by testing the very first hurdle novice programmers will encounter: assignment. The test results divided the students cleanly into three groups: 

44% of students formed a consistent mental model of how assignment works (even if incorrect!) 
39% students never formed a consistent model of how assignment works. 
8% of students didn't give a damn and left the answers blank. 

The test was administered twice; once at the beginning, before any instruction at all, and again after three weeks of class. The striking thing is that there was virtually no movement at all between the groups from the first to second test. Either you had a consistent model in your mind immediately upon first exposure to assignment, the first hurdle in programming-- or else you never developed one! 

The authors found an extremely high level of correlation between success at programming and forming a consistent mental model.....

http://www.codinghorror.com/blog/archives/000635.html