Days before the Tennessee Achievement School District is to announce whether it will take over five more Memphis schools next year, Vanderbilt has released a study suggesting the city’s low-performing schools would be better off in Shelby County Schools’ Innovation Zone.

The study, released Tuesday, shows that iZone schools have sizeable positive effects on student test scores, while the ASD’s effects are marginal. That means that students at ASD schools are performing mostly at the same low levels they likely would have had their school not been taken over by the state-run school turnaround district.

The study’s release comes at a time when the ASD is not only expanding in Tennessee, but several states plan to replicate it. ASD leaders have taken issue with the results, saying that it’s too early to draw decisive conclusions.

“The tough work of school turnarounds take time – the study itself says it takes up to five years for the reforms to take meaningful hold,” says a statement from the ASD leaders, whose first cohort of schools is in their fourth academic year in Memphis.

Researchers say it would be premature to scrap the ASD, which has been credited with creating a sense of urgency to improve the state’s worst academic schools. Students who have been in ASD schools for three years did see slight positive effects in test scores, and research typically shows that education reforms take a few years to show results.

That makes the iZone’s success in Memphis all the more remarkable, said lead researcher Ron Zimmer.

“I think the iZone schools are showing promising results that we can feel good about, and they showed them almost immediately,” he said.

Both the ASD and iZones were created when Tennessee won the federal Race to the Top grant in 2010 to improve struggling schools, among other things. Under the ASD model, low-performing schools are removed from their local districts into the ASD’s oversight, and most are placed with nonprofit charter operators to manage. Innovation Zone schools remain in the local district but, like charters, have the autonomy to hire and fire staff, overhaul their curriculums, give their teachers bonuses, and add time to the school day.

In Memphis, which has the state’s highest concentration of low-performing schools, the ASD currently oversees 27 schools, and the iZone has 18. School districts in Nashville and Chattanooga operate smaller iZones as well, and the ASD also oversees two Nashville schools.

The study did not explore why the iZone might be outperforming the ASD at this point. But another Vanderbilt study released last year showed that ASD schools had high levels of teacher turnover and student mobility in their first year — factors that often lead to a decline in test scores. Turnover leveled out in later years, which might be why they saw their scores rebound, and in some cases, surpass their pre-takeover levels.

Though the iZone managed to turn around schools without a drop in first-year scores, Zimmer said it remains to be seen if that success is financially sustainable. Already, the district is struggling to find money for the initiative, which is expensive to fund.

“There’s a question of how iZones are achieving this and is it sustainable?” Zimmer said. “That’s always an issue of reform. Is it scalable?”

ASD leaders said they believe that the data is inconclusive based on a young cohort and small sampling of schools and emphasized gains by ASD schools in both student proficiency and growth.

“Over the last three years, our schools outpaced the state in math and science, and our second- and third-year cohorts of schools averaged the highest possible growth rating (Level 5 TVAAS),” said the state district’s statement.

Editor’s note: This story updates a previous version with additional statements by ASD leaders.