Before beginning a study, a university researcher hypothesizes that when people are given free money, they are more likely to justify spending it on themselves when they believe that their own good choices led them to the money. For example, if someone does a good deed for an elderly neighbor and is then given money, he or she will be more likely to spend it on him or herself than on someone else. Conversely, if someone is given money after having done nothing noteworthy or praiseworthy in the course of his or her day, he or she is more likely to donate the money to a cause that is personally important. His argument is that when people feel positive about their actions, they seek to reward themselves in a tangible way.
In the design of the study, which of the following would most significantly weaken the reliability of his findings?