The math department recently purchased new learning software for their freshman math classes. The instructors want to use the learning software instead of a textbook. So they decide to study the effectiveness of the software. First they test the skills and concept mastery for a number of the freshman math students. Then they randomly divide the students into two groups. One group uses the learning software for a particular lesson and the other group uses the regular textbook. Then the instructors retest all of the students and compare the improvement in the skills and concept mastery for the two groups. What is the purpose of random assignment in this experiment