Research shows that, for a variety of reasons, including inequality in teacher pay, the best teachers tend to migrate to and concentrate in schools in higher-income areas.
30 Even within districts and in individual schools, where teacher pay is often uniform based on experience, factors beyond pay tend to  25  The share of four-year-olds in state-funded pre-K programs increased from 14 percent in 2002 to 27 percent in 2010 but has been 28 percent since. Head Start enrollments have been fairly steady since 2005.