Dr. Green was investigating the effect of a compensatory education program in mathematics for first-grade students. This study gave a standardized math achievement test to all first-grade students and...

Dr. Green was investigating the effect of a compensatory education program in mathematics for first-grade students. This study gave a standardized math achievement test to all first-grade students and administered the program to half of the students for the next 6 months; the other half of students were in the control group. During this 6-month period, a number of families moved and withdrew their children from this school. As it turned out, most of the children from families that moved were in the control group. It also turned out that most of the students who were withdrawn from school were poor math students. At the end of the 6-month period, Dr. Green again administered the standardized math achievement test to the treatment and control-group participants who remained in the study and found that the treatment-group students had increased their math score significantly more than the control-group students. Dr. Green concluded that the compensatory program was effective in remediating poor math performance.