Students enroll in a teacher’s classroom. Nine months later, they take a test. How much did the first event, the teaching, cause the second event, the test scores? Students have vastly different abilities and backgrounds. A great teacher could see lower test scores after being assigned unusually hard-to-teach kids. A mediocre teacher could see higher scores after getting a class of geniuses.

Thirty-five years ago, a statistician, William S. Sanders, offered an answer to that puzzle. It relied, unexpectedly, on statistical methods that were developed to understand animal breeding patterns.

Mr. Sanders died in March in his home state, Tennessee, at age 74, with his name little known outside education circles. But the teacher-assessment method he developed attracted a host of reformers and powerful lawmakers, leading to some of the most bitter conflicts in American education.

“In 1945, the United States government set off an atomic bomb.”

That’s how Mr. Sanders began telling me the story of his life, when we met several years ago.