At a Large University, a Group of Education Specialists Tested

At a large university, a group of education specialists tested the effectiveness of a new academic improvement course. Students seeking help at the university counseling center because of academic difficulties were asked to participate in this 4-week program. Only students who were judged to be deficient in reading comprehension and other study-related skills were chosen for the program. Students who sought help at the counseling center for emotional difficulties were included in the study as a comparison group. They received the usual treatment offered at the counseling center. A group of 30 students completed the academic-improvement program at the counseling center. Average test grades from the students' courses for the two groups were compared before (midterm exams) and after (final exams) the program. Analyses revealed that a statistically significant majority of the students were doing better in school after completing the academic-improvement program than before.
-What major threats to internal validity are controlled by this research design?