What are some possible controlled variables?

A researcher is interested in determining the effect of a new biology curriculum (NBC) on achievement of students as shown by the NABT exam. Students in 10th grade biology classes in a five county area of Southwest Florida were included in the study. One high school was utilized for each county involved. Using a t-test, the researcher reported a ';t'; value of 4.17, which was significant at the 0.04 level in favor of the students in the groups using the new curriculum. What are some possible controlled variables?What are some possible controlled variables?
Mmmm, the type of test, amount of students (used in test), the type of test used, the grade level tested..? Those are the ones that i could think of! good luck!