At a School There Are Two Different Math Classes for Children

At a school there are two different math classes for children of the same age. The two classes have different
teachers. The school principal is interested in gauging the effectiveness of two different teaching methods and
asks each teacher to try one of the methods. At the end of the semester both classes are given the same test and
the results are compared. In this experiment, what is the variable of interest? Give some examples of variables
which could be confounding variables.