How much last year's teacher affects this year’s test scores: Most value-added impact from teachers fades within one year, study finds

A recent trend in public education is to measure teacher quality based on how the students fared on standardized tests compared to previous years. If most of Mr. Green's current 5th grade students score at a higher percentile than they did as 4th graders, then Mr. Green gets what's called a high "value-added" rating.

In August, The Los Angeles Times shined the spotlight on this approach by publishing rankings for 6,000 L.A. schoolteachers based on value-added analysis.

The new study instead measured whether teachers like Mr. Green put students on a higher trajectory in the years to come. The researchers report that most of the gains from a highly rated teacher vanish quickly. In reading, 87 percent of the benefit fades after one year. In math, 73 percent of the gains fade after one year.

"People are looking for a silver bullet to fix public education," said BYU economics professor Lars Lefgren. "We've shown that the benefits are mostly transitory, so you don't want to sacrifice everything else you might value in a teacher just for value added to test scores."

Lefgren and fellow BYU economist David Sims co-authored the study with the University of Michigan's Brian Jacob for The Journal of Human Resources. Their analysis included eight years of data from 1.3 million student test scores in North Carolina schools.

While the news may sound depressing, the report offers this silver lining: The effect of having a crummy teacher doesn't last long either.

"You probably won't be scarred from having an incompetent teacher like me," Lefgren joked.