The value-added model examines longitudinal academic growth of students over time. It aims to measure each student's growth and control for external factors by using the student as their own control. Research has found significant variability in teacher effectiveness, with top-performing teachers helping students make the largest learning gains. Studies show students assigned to highly effective teachers for multiple years have much higher achievement and pass rates than those assigned to less effective teachers for the same period. The research demonstrates teacher effectiveness has the greatest impact on student academic growth.