To test the value of the study guide, Professor Lewis gives a free copy to the students sitting in the first two rows of class. Later she finds that these students got better grades than the students in the back of the room (who didn't get study guides). What is wrong with this experiment?

Question

Quizplus · Accepted Answer

The lack of random assignment to groups means that there could be other variables (such as motivation or intelligence levels) that are causing the difference in grades between the two groups, rather than just the study guide itself. Without random assignment, the experiment cannot definitively show that the study guide is the cause of the better grades.

To Test the Value of the Study Guide, Professor Lewis