Several high schools in a large metropolitan city recently started a "study buddy" system to help children who are having difficulty in school. The program pairs struggling ninth and tenth graders with 11th and 12th graders who are doing well in school. The "buddies" get together twice a week after school on the school grounds. The younger students can ask for help in any subject. Some of the older students excel in math, others in English, and others in various subjects. Therefore, the program provides tutoring help in all subjects to any of the students seeking it.

At the beginning of the year (September) 80 ninth and tenth graders began the "study buddy" program. Approximately half (42) of the 80 students continued to attend the program throughout the school year. The mean posttest grade point average (GPA) of the 42 who finished the program was significantly higher than the mean pretest GPA of the 80 who started the program. The 42 students also reported more positive attitudes toward attending school, and the assistant principal noted far fewer behavior problems among those students at the end of the semester.

1.What are some possible threats to validity in this study?