Correction: An earlier version of this story incorrectly referred to TNTP, the non-profit organization that released a new study about professional development for teachers, by its former name. The story has been updated.

A new study of 10,000 teachers found that professional development — the teacher workshops and training that cost taxpayers billions of dollars each year — is largely a waste.

The study released Tuesday by TNTP, a nonprofit organization, found no evidence that any particular approach or amount of professional development consistently helps teachers improve in the classroom.

“We are bombarding teachers with a lot of help, but the truth is, it’s not helping all that much,” said Dan Weisberg, TNTP’s chief executive. “We are not approaching this in a very smart way. We’re basically throwing a lot of things against the wall and not even looking to see whether it works.”

Researchers examined three large school districts as well as one network of charter schools. They looked at professional development programs at all the schools and teacher performance data over several years, and they surveyed 10,000 teachers and interviewed more than 100 administrators. They identified teachers who improved their job performance and tried to figure out what experiences they had that differed from teachers who were stagnant. To determine if a teacher had improved, researchers analyzed multiple measures — evaluation ratings, classroom observation and student test scores.

And they didn’t find many answers.

“When it comes to teaching, real improvement is a lot harder to achieve — and we know much less about how to make it happen — than most of us would like to admit,” Weisberg said.

The school districts that participated in the study spent an average of $18,000 per teacher annually on professional development. Based on that figure, TNTP estimates that the 50 largest school districts spend an estimated $8 billion on teacher development annually. That is far larger than previous estimates.

And teachers spend a good deal of time in training, the study found. The 10,000 teachers surveyed were in training an average of 19 school days a year, or almost 10 percent of a typical school year, according to TNTP.

“The bottom line is, they’re spending a lot of money on this and it’s such an appealing idea — take your existing teachers and just make them better and everybody is better off,” said Eric Hanushek, an economist at Stanford University’s Hoover Institution. “But this report finds that, on average, it doesn’t do much.”

The findings echo two recent federally funded studies, which concluded that current approaches to teacher training have no significant effect on performance.

“At the federal level, we spend $2.5 billion a year on professional development,” Education Secretary Arne Duncan said at a teachers town hall meeting in 2012. “As I go out [and] talk to great teachers around the country, when I ask them how much is that money improving their job or development, they either laugh or they cry. They are not feeling it.”

School districts have failed to adequately scrutinize the quality of their training programs, Hanushek said.

“School districts just have to take it more seriously,” he said. “They have to manage the use of professional development, evaluate its usefulness and dump the bad stuff. Which is a common problem in schools. They add something, and if it’s not working, they add something on top of that. They’re good at adding, not as good at taking away.”

In the TNTP study, about one-third of teachers — 3 out of 10 — improved over a two-to-three-year period after participating in training while 20 percent got worse, as measured by teacher evaluations.

The study also found that school districts are not helping teachers understand their weaknesses. Fewer than half of the teachers surveyed agreed that they had weaknesses in the classroom while more than 60 percent of teachers who earned low performance ratings gave themselves high grades.

“There is no doubt that there are initiatives that are probably producing positive impacts,” Weisberg said. “But it’s not helpful if you don’t know what they are. It is really important for school systems to begin to set goals and measure impacts against those goals. If we do that, we’re going to be so much smarter than we are now.”