Beth is interested in studying the effects of movies on peoples' attitudes toward the mentally ill. she obtains a sample of college students who have never seen one flew over the cuckoo's nest. half of the students watch the movie and half do not, and then she measures their attitudes toward the mentally ill. she is worried that if participants have a loved one who is mentally ill, they may have different attitudes from everyone else. she decides to give a pretest to determine if people have family members with a mental illness to make sure that there are equal numbers of such people in both the experimental and control groups. beth is using