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The interactive tool shows that

Racial segregation tends to concentrate black and Hispanic children in schools where most of the students come from poor families because of the persistent connection between race and income in the United States. And those high-poverty schools provide fewer opportunities than schools that are more affluent.

How stark are the wealth disparities? In 2016, the median wealth of a white family鈥攁ssets minus debt鈥攚as $171,000, based on calculations of federal data conducted by the Urban Institute. For black families, median wealth was a little over $17,000. For Hispanic families, it was just shy of $21,000.

鈥淚t鈥檚 the difference in the poverty composition that is most predictive of the achievement gap,鈥� said Sean Reardon, a professor at the Stanford University Graduate School of Education and the study鈥檚 lead author. 鈥淭hat鈥檚 the mechanism by which segregation is harmful.鈥�

To calculate school effectiveness and achievement gaps, the researchers turned to a data archive they created using federal sources such as state test scores as well as results on the National Assessment of Educational Progress. Segregation data were derived from an annual survey of all schools in the country. Poverty was measured by looking at the percentage of children in a school who are eligible for free- or reduced-price lunches. All those data have been compiled into an interactive online tool that the public can use to rate and rank schools and districts nationwide.

In their analysis, the researchers found that highly segregated districts had sizable achievement gaps, and the rate of the gap grew faster as students progressed from 3rd to 8th grades.

Comparing Districts

To see this in action, consider the school districts in Atlanta and Baltimore. Both are highly segregated by race and both have large test-score gaps between black and white students. Atlanta is about 75 percent black and 15 percent white; Baltimore鈥檚 student population is about 82 percent black and 8 percent white.

But in Atlanta, the test-score gap between white and black children is nearly five grade levels, and in Baltimore, the test-score gap is two grade levels, according to the tool developed by Stanford.

The interactive tool shows that black students in Baltimore attend schools that are relatively more affluent than black students in Atlanta. In Baltimore, black schools are 15 percent poorer than the schools attended by white students. In Atlanta, those schools are 56 percent poorer than those attended by white students.

In contrast, Detroit, which is 83 percent black and about 2 percent white, has an achievement gap of less than a grade level between black and white students. But both groups are scoring more than two grade levels below national averages, and both groups are attending schools classified as high poverty.

鈥淲hether your child is white or black, that [high-poverty] school is likely to be much less effective,鈥� Reardon said.

The study adds to a growing body of research about the effects of segregation on academic achievement.

A 2017 study by David Leibowitz, an analyst for the Organization for Economic Cooperation and Development, found that residential segregation increased in districts after desegregation orders were lifted.

When districts were no longer required to integrate their schools, dropout rates rose by 1 percentage point for black students and by 3 percentage points for Hispanic students.

And, in a 2017 study of New York schools, researchers found that gaps between white students and black and Latino students in math- and reading-test scores, as well as gaps in graduation rates, were all smaller in the city鈥檚 most integrated schools, compared with the city鈥檚 most racially segregated schools.

In the city鈥檚 most segregated schools, white students were 23 percentage points more likely to graduate from high school than black students and 29.4 percentage points more likely to graduate than Latino students. By contrast, in New York鈥檚 most racially and ethnically mixed schools, the Latino-white graduation gap was only 4 percentage points, and black students graduated statistically on par or even slightly more often than white students.

Understanding Why

So what is it about high-poverty schools that stifles students鈥� academic growth?

That鈥檚 a harder question to answer, the researchers concluded, possibly because the proxies for school quality that can be quantified鈥攖hings like the existence of gifted programs, the number of skilled and experienced teachers, or the use of more challenging curricula鈥攁re poor gauges for evaluating a school鈥檚 quality. High-poverty schools have fewer of those resources, but that does not seem to be associated with achievement gaps.

The researchers also examined whether poor schools have large groups of students with lower test scores compared with schools with more white students. That clustering could mean that teachers can鈥檛 concentrate on rigor, or that there are differences in norms around achievement, the researchers hypothesized. But those elements also didn鈥檛 explain the gaps, the papers found.

Even though the analysis did not pin down the mechanisms that lead to achievement gaps, it does show poverty is the cause, rather than a school鈥檚 racial makeup. If it were possible to address academic-achievement gaps while leaving racial and economic segregation intact, Reardon said, some school district or community would have figured it out by now.

鈥淚t may not be possible,鈥� he said. 鈥淚t certainly isn鈥檛 easy.鈥�