1. A research methods instructor wants to know if having students conduct their own research study as part of the course increases their understanding of major concepts in the course. To investigate this, she gives two sections of her course a pretest on the course concepts. She then gives one section a research study assignment for the course but does not give this assignment to her other section. At the end of the semester, she gives a post-test to both sections on the course concepts and compares the difference in the pretest-posttest scores as a measure of learning for the two sections of her course. The section with the research study assignment shows more learning.

Explain why the instructor cannot be sure that the research study assignment caused more learning in this study.