Dr. Brown is a developmental psychologist who studies how children learn mathematical principles. Her current interest is in studying how children learn multiplication. She recruits 64 third-graders who have not yet learned how to multiply numbers to participate in her study. She is curious as to whether different teaching styles lead to better mastery of the subject. One condition (A) involves having students being taught multiplication by a teacher. The other condition (B) involves having students being taught multiplication by an advanced fifth-grader. The dependent variable will be scores on a multiplication test.

Imagine that Dr. Brown knows that interest in math is related to math performance, and she is worried that this individual difference variable may not get equally distributed between her two conditions through random assignment.

A) What change can Dr. Brown make to her research design to enhance the internal validity of her study?

B) Describe the specific steps the researcher will take to implement this change:

C) How does this technique improve internal validity? Explain in detail to demonstrate your knowledge.