Since 2012, the Denver school district has given extra money to its low-performing schools to help accelerate improvement — currently, up to $1.7 million per campus over a five-year period. A new study suggests the strategy is working, with students at the targeted schools making bigger academic gains than did similar students elsewhere.

“We know about how we do, but it was nice to see an external validation of our approach,” said Denver Public Schools Deputy Superintendent Susana Cordova.

The study was published by the Center for American Progress, a progressive think tank headquartered in Washington, D.C. Author Samantha Batel compared the state literacy and math test scores of students at Denver schools that got the extra funding, which the district calls “tiered support,” with the scores of students at other schools across Colorado.

But she didn’t only use raw test scores. She also looked at how much academic progress students make in a year, as measured by what Colorado calls “growth scores.”

Those are percentiles, and they’re calculated by comparing a student’s raw test score with the scores of students who performed similarly to them in previous years. A growth score of 99 means a student did better than 99 percent of students with similar test score histories.

Batel found that the growth scores of students at 35 Denver elementary schools that got extra funding were higher by 5.2 percentiles in math and 2.5 percentiles in literacy than growth scores at 117 Colorado schools with similar student populations in terms of race and family income. In other words, the Denver students made more academic progress year over year.

When Batel further controlled for factors such as race and family income — which effectively made the schools’ demographics the same instead of just similar — the Denver students fared even better, outperforming the others by 7.8 percentiles in math and 4.1 percentiles in literacy.

Hispanic students attending Denver schools that got extra funding had especially high growth scores compared with their peers around the state, Batel found. The same was true for students receiving free or reduced-price lunch, a proxy for poverty.

Denver separates its district-run schools into three tiers based on student test scores, school ratings, and other factors such as teacher turnover. The most struggling schools are placed in the “intensive” tier, schools that show some warning signs of struggle are put in the “strategic” tier, and schools that generally serve students well are put in the “universal” tier.

Schools in the intensive and strategic tiers get extra funding, and they tend to spend it in three areas, Cordova said. One is extra support for students, which could mean hiring more specialists or extending the school day. Another is more time for teachers; most schools will pay their teachers to come back a week earlier in the summer to prepare for the school year ahead. Yet another is more training for both teachers and principals.

The district has explored other ways to support struggling schools that don’t cost extra money, too, Cordova said. One example: Making sure its technology department prioritizes setting up the email accounts of new teachers at intensive- and strategic-tier schools, which tend to have higher staff turnover and more new hires every fall.

In addition to the study, Denver Public Schools’ own data show the tiered support approach is making a difference, especially at schools that get the most extra money. Students at schools in the intensive tier made larger gains on state tests than have the district’s students as a whole.

However, those students’ test scores still lag far behind the district average. Batel admits that while her study shows that those students made above-average gains, it doesn’t offer an indication of whether the gains are enough to bring the students, most of whom are black and Hispanic and come from low-income families, up to grade level.

Denver Public Schools has wide test score gaps between students of color and white students, and between students from poor families and students from wealthier ones.

The study has other limitations, as well. For one, it doesn’t account for whether the 117 Colorado schools used as a comparison were getting extra funding from their districts, too.

The study also attempts to measure the impact of the extra funding by looking at whether the schools’ ratings improved over time. They did, but that’s arguably an imperfect measure, given that the way the district calculates those ratings has changed over time.

And finally, even Batel herself notes that it’s impossible to attribute the test score gains solely to the fact that the schools received extra funding — particularly since Denver Public Schools is known for trying several different improvement efforts all at once.

“However, the gains made by the entire district and by the schools receiving preventative or more intensive supports are suggestive of momentum in the right direction,” the study says.