Suppose that a school enforces a new policy to prevent bullying. The school also forms a very diverse committee of students, faculty, and parents to monitor the effectiveness of the policy. During the next year, the committee successfully collects very thorough information about overall student quality of life. At the end of the school year, it seems that there was no increase in the number of bullying events. Nonetheless, when trying to determine whether the policy has been effective at preventing bullying, the biggest challenge still facing the committee is: