Cross levels (from Dr. G.`s Decision Tree) refers to subjects (from the same group) in an experiment being subjected to more than one treatment.

Example:  Subjects were given 10 mg. of statistoprazole, then they took a statistics exam and their scores were measured.  Immediately after the first test they were given another 20 mg. and given another exam and again their scores were measured.  These subjects crossed levels (of treatment amounts) because the first dose of statistoprazole might have affected the second dose. 

AKA: repeated measures