School districts need to get smarter about retention strategies because the strongest teachers are just as likely as weak teachers are to leave their schools after five years, according to a study by the New Teacher Project.

“Only 10 percent of teachers are leaving, each year,” said Timothy Daly, president of the project, which trains city teachers. “The problem, though, is that we lose so many great teachers, especially early in their career, that half of them are gone by year six.”

The study looked at four urban districts, including New York City. Its authors say they relied on student test scores to identify the top 20 percent of teachers, whom they call the “irreplaceables.” These are teachers whose students consistently make the most progress on state exams, year after year. On average, they help students learn two to three additional months’ worth of math and reading compared with an average teacher. And they get high marks from their students.

The report recommends teacher merit pay and better teacher evaluation systems, two hot-button issues contested by the unions representing teachers and principals. The report also says school districts should evaluate principals partly based on how well they do in retaining the best teachers.

In its survey the New Teacher Project found two-thirds of the best teachers had not been asked to stay at their schools by the principal, which Mr. Daly called “pretty shocking.” The study also says principals rarely counsel the low-performing teachers to leave.

Those conclusions were surprising to the union that represents New York City principals, the Council of School Supervisors and Administrators.

“My experience says there’s no research, or very poor-quality research, to support the statement in the report,” said Chiara Coletti, a C.S.A. spokeswoman. “Never have I met a good principal who didn’t care, often to the point of obsession, about retaining good teachers.”

The city teachers’ union, the United Federation of Teachers, also took issue with the report’s reliance on student test scores, known as value-added data. New York City used test scores to create teacher data reports, which were then used in tenure decisions. But the data reports were criticized by the union for having wide margins of error and mistakes.

The union’s president, Michael Mulgrew, embraced the report’s other recommendations, though his support came in an e-mail laced with sarcasm:

“Better working conditions? More support for teachers? A career ladder? Reducing teacher attrition? It’s hard to believe that these recommendations — long supported by the U.F.T. — are coming from a group supported by the Walton Foundation,” he said.

The Walton Family Foundation has been criticized by the union for supporting charter schools, and for its conservative positions.

Not surprisingly, the Bloomberg administration applauded the report’s conclusions. The New Teacher Project runs the Teaching Fellows, an alternative certification program for recruiting and training city teachers.

Mayor Michael R. Bloomberg and Chancellor Dennis M. Walcott issued a joint statement saying the report “confirms that school districts across the country must do more to keep great teachers in our classrooms.

“That’s exactly why we have offered to add a $20,000 annual stipend to the salaries of teachers who are rated highly effective for two consecutive years. Earlier this month, President Obama also outlined a bold plan to pay exceptional teachers an annual stipend. Unfortunately, the United Federation of Teachers continues to stand in the way of these incentives, which New York City public school teachers deserve,” they said. “It’s time for the union to work with us so that our irreplaceable teachers can take full advantage of these and other opportunities.”