Dr. Harrison wants to test the effectiveness of the support group he runs for undergraduates who have drinking problems. He recruits a group of students who have been referred to the counseling center. He randomizes them to two groups — atreatment group and a control group. The treatment group attends 10 structured support group sessions at 8:00 a. m. on Fridays that are facilitated by Dr. Harrison.
The control group attends 10 unstructured meetings at 3:00 p. m. on Wednesdays that are facilitated by members of the group. Several of the participants in the treatment group stop attending the group after just a couple of sessions. All of the control group members attend their group regularly. At the end of the 10 weeks of groups, Dr. Harrison measures drinking behavior of the people who are still attending the groups. He finds that the drinking behavior of people who attended all 10 of the structured group sessions is less than the drinking behavior of people who attended all 10 of the unstructured group sessions. What kind of threat to internal validity should Dr. Harrison be concerned about?