Suppose that your hypothesis predicts that if a subject experiences a high level of stress, he or she will perform more poorly on a problem-solving task. in an experiment, you attempt to create a high level of stress for the "high-stress" group by yelling things like, "hurry! go faster! " for the "low-stress" group, you calmly say things like, "don't worry. take your time." you record how many problems each subject solves, and you also measure each subject's level of stress using a questionnaire that has been used by many previous researchers to measure stress. in this situation, the stress questionnaire represents: