At a time when scores are about to be used for high-stakes decisions in how to improve Tennessee’s schools, gaps in the state’s data and uncertainty about how scores were derived have left Memphis officials wondering how to interpret the torrent of information.

Last year’s chaotic state testing, which led to the cancellation of the state’s test for grades 3 to 8, left a crucial gap in the data meant to help make decisions about schools and teachers.

School leaders have also said they were puzzled by the state’s methodology in reaching the so-called growth scores upon which districts and schools are judged — particularly by how they arrived at the Memphis district’s low score.

Even those who are paid to sift through the data say they are having trouble getting answers to questions about the growth scores, known as TVAAS. Bill White, chief of planning and accountability for Shelby County Schools, conceded to board members last week that he didn’t know the ins and outs of the complex formula and the changes meant to compensate for the missing data.

“I have personally never been shown all the mathematics behind our data and how this works,” he told board members. “I do know that it has been peer-reviewed and vetted and it’s essentially been held up among those statisticians. But there is a lot that goes on behind the scenes that no one has been able to walk us through.”

The confusion has renewed skepticism about the state’s value-added model, which is supposed to help officials identify the impact that schools and teachers have on student performance. The system relies on the state’s data measuring student growth in districts.

Part of the problem is last year’s botched testing, which is having multiple ripple effects throughout the state.

This year, growth scores are comparing 2016-17 test results with the 2014-15 school year, the most recent data available. That throws a wrench in how to assess which school or teacher is responsible for a child’s growth over a two-year period. And for elementary schools, that means there is no data for fourth graders this year since testing in third grade, the first year students take state tests, was canceled.

In addition, one subject was dropped entirely from TVAAS calculations because social studies questions were a trial run for elementary and middle schools students and did not count.

Statisticians for the most part have figured out how to calculate growth even when a state transitions to a new test. But the missing data creates a whole other host of challenges the revisions attempt to account for.

One Memphis charter leader said he still isn’t quite sure how his school even got a score since last year his highest grade level at the school was third grade, the first year of testing.

“It’s such a convoluted formula, it’s hard for us to understand. We’re not sure how we got (our score),” said the charter leader, who declined to be named because he was still seeking answers from the state.

Damian Betebenner, a senior associate at Center for Assessment that regularly consults with state departments, said missing data on top of a testing transition “muddies the water” on results.

“When you look at growth over two years, so how much the student grew from third to fifth grade, then it’s probably going to be a meaningful quantity,” he said. “But to then assert that it isolates the school contribution becomes a pretty tenuous assertion… It adds another thing that’s changing underneath the scene.”

At the same time, TVAAS scores for struggling schools will be a significant factor to determine which improvement tracks they will be be placed on under the state’s new accountability system as outlined in its plan to comply with the federal Every Student Succeeds Act. For some schools, their TVAAS score will be the difference between continuing under a local intervention model or being eligible to enter the state-run Achievement School District. The school growth scores will also determine which charter schools are eligible for a new pot of state money for facilities.

The state has data analysts based across Tennessee to help districts with their questions and provide data simulations for the complex formula that has been replicated in other states.

“Of course, the reason it is complex is because we want it to be fair for educators and therefore capture as much data and nuance as possible – which is discussed at length in the technical documentation,” said a state department spokeswoman.

The state has also published an overview video of how the formula works and details on the recent changes in a 46-page, formula-packed document from SAS, the private company that calculates teacher and school scores for the state.

But as far as knowing how the state gets from A to Z, White said he still has questions.

“I’ve had some questions about getting access to certain data myself,” said White, who routinely interprets data for the district. “We would like a lot more access to what goes into TVAAS.” (He later declined to elaborate.)

He’s not the only one. When the Tennessee Education Association unsuccessfully sued Knox County Schools over its use of TVAAS in awarding teacher bonuses, access to data on how the scores were calculated was central to the association’s argument that the district denied teachers due process, said Rick Colbert, TEA’s general counsel.

When Colbert attempted to subpoena technical documents on the calculations, SAS blocked it partially because the request would divulge “trade secrets.”

“When they’re called upon to defend it you get a lot of general statements but you can’t get a lot of information to see if you can back that up,” Colbert said. “There’s so much about TVAAS that can’t be explained.”

Board member Mike Kernell called it a double standard and asked White last week if the district could request a demonstration of the complicated formula.

“I think the state department of education ought to show its work if they’re asking children to show their work,” he said.