Researchers are interested in whether feedback on one math test influences motivation for a second test. Middle school students meet individually with an experimenter and are given a math test. After grading the test, the experimenter gives every student positive feedback by saying, "You did well on the test." Half the students are also told, "You must have worked very hard," and the other half are told, "You must be very smart." Experimenters found that students who had been told they must be smart were reluctant to take a second math test and were more likely to choose a test they knew would be easier. Participants who were told they must have worked hard were eager to take a second test and happy to choose a challenging test. What is the independent variable in this study?