Dr. Wickersham wants to examine how light affects participants' performance on a counting task. So she has two groups, one that works under dim light and one that works under bright lights. She finds that those in the dim-light condition perform better. However, one of her research assistants admits that he gave those in the dim-light condition extra time to complete the task. Now Dr. Wickersham is not sure whether those in the dim-light condition did better because of the lighting or because of the extra time they received. The extra time on the test is an example of a(n) _____ variable.