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 helped 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, unidentified school districts as well as a network of charter schools.

They looked at professional development programs at all the schools and teacher performance data over several years and surveyed 10,000 teachers and interviewed more than 100 administrators. They identified teachers who got better at their jobs 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.

The school districts that participated in the study spent an average of $18,000 per teacher annually on professional development. Based on that, 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 conservative Hoover Institute. "But this report finds that, on average, it doesn't do much."