Dr. Mientka designs an experiment. He randomly assigns his participants to 2 groups, pretests them on general intelligence, and then proceeds with his treatment (one group participates in a self-esteem-building program, and the other group participates in a resiliency/self-regulation-building program). These programs take 6 months to run. Then he posttests the participants in each group with the same test that was his pretest. During the 6-month programs, 17 of his participants leave the study for a variety of reasons. This means that Dr. Mientka's results might be affected by: