theless it is important to identify possible negative effects of mobility, both for individual students and the schools into and out of which they transition.
To address this need, Hanushek and his colleagues attempted to hold all other factors (e.g., family and neighborhood characteristics) constant and examine the independent effect of changing schools. They developed a model that uses measures of achievement growth to examine the different ways in which mobility might affect individual students and schools.1 Achievement is the product of many factors, so they made a few assumptions to simplify the analysis. First, they assumed that achievement during the school year prior to a move was not unusually bad or good. Second, they assumed that students are generally on a growth path, and that this path is likely to follow a particular trajectory, assuming a constant school quality from grade to grade. That is, if a student is learning at grade level, he or she is likely to continue at that level after moving, assuming the new school is of similar quality. Finally, they assumed that the disruption that caused the move (e.g., divorce, economic upheaval) lasted only for the year in which the move took place. These assumptions allowed them to isolate any changes in school quality that students experienced when they moved.
Hanushek and his colleagues applied the model to data from the Texas Schools Project, which provided attendance and mathematics achievement data for three cohorts of students in grades 4 through 7. For the general student population, including students who move for different reasons, they found that the move itself had little effect on the students’ academic trajectory (past the disruption in the year of the move). In other words, Hanushek said, “if they’re in a bad situation, the move has very little marginal effect on their bad situation.” More challenging was to discover whether high mobility has a measurable effect on peers, teachers, and the quality of the schools into and out of which students move. To do this, they compared student achievement among, for example, fourth graders in consecutive years, and used the differences in mobility rates “to see whether mobility shows up in differences in achievement, other things being equal.” They found that “higher student mobility in a school during the school year really hurts everybody, and it hurts people in a fairly dramatic way.”
Hanushek emphasized that for people in high-mobility schools these effects persist throughout their school careers. Moreover, African American students in the Texas sample had much higher mobility rates than other children, and they also tended to go to schools with much higher mobility. Hanushek and his colleagues estimated that the difference in mobility rates between white and African American students in Texas