Holding back struggling thirdgraders helps them advance academically more than allowing them to move on to the fourth grade, a new study by the Manhattan Institute found.

The study, out today, analyzes the performance of students in Florida, which enacted a policy to end the so-called social promotion of failing students a year before Mayor Bloomberg, with Schools Chancellor Joel Klein, created his plan for third-grade retention in New York City.

It compares the third-graders who performed poorly and were held to the policy in the 2002-03 school year to the low-scoring students who took the test in the 2001-02 school year and would have been held to the policy had they been born a year later.

The researchers, Jay Greene and Marcus Winters, found that a year later, the students who were held back under the new policy outperformed the students who would have been held back. The difference was 2 percentiles in reading and 4 percentiles in math.

They also compared students who were held back in the second year of the study to those who scored below the test cutoff but were allowed to advance to fourth grade. A year later, the average reading scores of students who were retained in third grade were 4 percentiles above those of the students who were allowed, despite low thirdgrade test scores, to advance. In math the difference in average test scores was 10 percentiles.

“These results are very supportive of the policies that Klein and Bloomberg have adopted,” Mr. Greene said. “While this is only one year’s worth of evidence, these initial results support what they’re doing.” He said the statistical analyses he and his colleague did backed up the idea that “students who have not mastered third-grade skills do not prosper when they’re placed in the fourth grade.”

Mr. Bloomberg and Mr. Klein implemented their plan to hold back thirdgraders who scored at the lowest level on citywide math and English exams last spring. This fall, Mr. Bloomberg announced the program had been successful for third-graders and said the policy would be extended to fifth grade.

Officials of the city Department of Education were pleased with the results of the study.

“The findings are encouraging in that the Florida students subject to the retention policy achieved greater gains in both English and math than those not subject to the policy, and students who were retained performed better in English and math than similar students who were promoted,” said an agency spokesman, Keith Kalb.

“We are confident that our third- and fifth-grade policies and our focus on working individually with lower-performing students will help our children learn more and achieve more in higher grades.”

Mr. Greene and Mr. Winters decided to investigate the results of Florida’s grade-retention program to find out whether it would be harmful or helpful to hold back students with the lowest scores on standardized exams, in Florida and elsewhere. Currently, nine states and some additional big cities, including New York, Chicago, and Philadelphia, have adopted mandatory-promotion tests.

Despite the rise of policies to end social promotion and the increasing controversy surrounding the issue, Mr. Winters said there has been little reliable research on their efficacy. He said the one study of a similar program in Chicago compared students who scored just below the score cutoff to students who scored just above the cutoff – something he said skewed the results.

The researchers said that not only did their analysis of Florida thirdgraders find that struggling students who were held back progressed academically more than their peers who were promoted, but it found that Florida handed out too many exemptions.

Before the policy was enacted, just 9% of the students who fell below the testing cutoff established by the policy were held back. After the policy was enacted, 60% of children who scored below the threshold were held back.

Mr. Greene said if school officials were handing out the ideal number of exemptions, both the children who were held back and the children who were promoted would excel academically.

“While we definitely want ways for students to be promoted without the test score, we should be careful about giving out those exemptions,” Mr. Marcus said. “Many of the students who received exemptions and were passed on probably would have benefited from another year in the third grade.”