Bill Sanders is a 72-year-old retired mathematician living an hour outside of Nashville who for decades has sparked fierce debate among teachers around the country. And he continues to. The reason? He devised a way to assess teacher performance that many educators say is unfair, though it’s been embraced by many states.

Sanders is self-described numbers guy, and he’s used his math chops to delve into questions across the globe.

“The first estimate of the panda population in China was made in my office,” Sanders said recently at his home, which sits at the end of a cul-de-sac in Columbia, Tenn.

Another feat: He figured out that the fish-killing red tides in the Gulf of Mexico were not due to development along the coast, as everyone had thought, but due to mining in Central Florida.

In the late ‘90s, Sanders was something of an education policy rock star, traveling more than 30,000 miles in a single year to talk about his way of tracking a teacher’s impact.

How did Sanders become so involved in education?

It all started in 1982. He was then a professor at the University of Tennessee. Sanders stumbled upon a newspaper article suggesting there’s no proper way to hold teachers accountable based on test scores. He said nonsense.

What started as a personal challenge to prove the conventional knowledge wrong quickly turned into a career.

“Basically, all I was trying to do is: here’s the statistical methodology that solves the problem that some of the critics are talking about. That was my whole attempt. Here, here. You might not know about this. Here.”

Soon enough, every public school teacher in the state became very aware of his model. It’s called TVAAS, or the Tennessee Value Added Assessment System. Today, some version of his model is used in Pennsylvania, Ohio, North Carolina and parts of Texas.

First introduced to help teachers by showing how they can improve, many now say it’s being used against them. Yet Sanders says, unapologetically, weeding out bad teachers is a must.

“Particularly math. Math is so cumulative. That if kids get behind at any step along the way, it snowballs. They wind up with a bad attitude about it: I never liked it. I’m no good at it. And it absolutely starts at getting the wrong sequence of math teachers, etc.”

Here’s how the TVAAS system works:

Each student’s standardized test results are compared over several years. And the same is done for every student in the classroom. The aim of Sanders’ equation is to see what value the teacher is adding to each student.

Yet here’s the rub: Students move, get sick, change schools. So a lot of student data is missing. Sanders came up with a way to make an educated guess about filling in that missing information based on a very complicated calculation. Skeptics call this his black box.

“I’ve heard this a zillion times. Sanders has a black box, and nobody knows what’s in it?” he said. “My answer is: How many copies or reprints do you want?”

Granted — though nothing about it is secretive — to really understand the equation, you need to understand so-called mixed models and covariant structures.

One criticism has been that since the formula is so complicated, teachers have to put faith in its accuracy. Skeptics say that might be OK for religion, but not when you’re talking about education.

And more than twenty years after it was developed, it’s still riling teachers. The country’s largest teachers’ union recently elected a new president who called assessments based on Sanders’ model the “mark of the devil.”

But Sanders said he’s heard it all. “The unions now are having a hard time, because it’s no longer the little man from Tennessee.”

Inspired By The Dairy Farm

Sanders was raised on a dairy farm near Shelbyville. He went off to the University of Tennessee and earned a PhD in biostatistics and quantitative genetics. Thus, statistics, as it related to raising animals, resonated with him.

“If you have a choice of buying Bull A, compared to Bull B, of which one is more likely to produce daughters that will give more milk than the other one.”

Bulls die and get sick and so have missing data over time. It’s the same math quandary he tried to solve in classrooms.

“The analogy is: you measure the effectiveness of teaching on the progress the students make, as opposed to some characteristic directly of the teacher.”

He’s explained his model privately to Bill and Melinda Gates, and they in turn have donated millions of dollars to programs that use his model.

In Tennessee, his method is under attack.

Teachers have filed federal lawsuits alleging that the evaluation is unfair. They say it fluctuates dramatically year to year and doesn’t take into account other factors about students’ lives — like poverty and race.

Sanders rebuts that by saying: those factors are implied in the students’ data. He says nobody else has put forth a better way to hold teachers accountable. This is what’s at stake he says:

“Large chunks of the student population do not have the educational opportunities that they need to have more choices in life.”

But should the results be tied to teacher pay and tenure decisions? He says it’s possible to take the results too far, but he was very hesitant to talk about policy decisions.

“It’s not for the statistician to basically say: this is what the policy ought to be,” he said.

He says he tries to stay above the political fray. But in many ways, he’s right at the center of it.

Bill Sanders, the creator of TVAAS, at his home in Columbia, Tenn.