Dr Paddock Is a Counseling Psychologist Who Is Interested in Decreasing

Dr. Paddock is a counseling psychologist who is interested in decreasing adjustment issues in first-year college students. She is curious if having students create collages of their first few weeks of school and then mail them home will help students feel they have integrated their new life with their old and, as a result, will help them feel less homesick. She samples a group of 100 incoming college freshmen at her university and measures how homesick they are during the first week of school. During week 4 of school, she has them make the collage and send it home. During week 7 of school, she measures their homesickness again. She notices a significant reduction in the amount of homesickness from the pretest to the posttest and concludes that her treatment is effective.
Imagine in Dr. Paddock's study that only 90 of the original participants completed the measure of homesickness during week 7 (10 participants had left the university and were unavailable). What kind of threat to internal validity does this pose? How does this affect her conclusion that her treatment for homesickness worked?