Second-grade students who attended full-day kindergarten outperformed other second graders who attended half-day kindergarten in standardized reading assessments, according to a study released Monday by the Clark County School District.

School system officials said the results from the interim study provide the first glimpse of tangible evidence that indicates full-day kindergarten is beneficial to Southern Nevadan students in the long term.

But critics contend that the study is biased and the phrase "long-term" should not be used when describing students not much older than kindergartners.

The district's study assessed 1,233 students in the second grade from Sept. 11 to Sept. 22 in 2006. About half of the students in the study attended full-day kindergarten while the other half attended half-day kindergarten.

Researchers tested students with a reading program that indicates what a student's reading level should be in the second grade.

Students who went through a full-day kindergarten program on average scored 3.1 percentage points better than half-day kindergarten students.

At-risk students, including those whose primary language isn't English and low-income students, fared even better. Students with language barriers who had full-day kindergarten performed nearly 8 percentage points higher than half-day students who had language barriers. That figure was similar when measuring low-income students who had full-day kindergarten versus their counterparts who had half-day kindergarten.

Lauren Kohut-Rost, the district's deputy superintendent of instruction, said the results were not surprising, and they demonstrate that every elementary school student in Nevada deserves the opportunity for full-day kindergarten.

"We need the time that full-day kindergarten affords to fully work with each child and target their educational needs," Kohut-Rost said.

She noted that full-day kindergarten students attend school for six hours a day compared with about 2 1/2 hours for half-day kindergarten students.

The district offers at least one class of full-day kindergarten at 109 of its 199 elementary schools. Students in the study came from 22 schools that offered full and half-day kindergarten.

The study was released the same day Silver State politicians convened in Carson City to launch the legislative session.

Democrats and Republicans disagree on the merits of full-day kindergarten and whether it should be extended to all elementary schools in Nevada. Democrats support extending the program to all elementary schools while Republicans and Gov. Jim Gibbons generally favor keeping the program at low-income schools while waiting for more conclusive evidence.

State Sen. Bob Beers, R-Las Vegas, called the study "laughable." He said he's not surprised that full-day kindergarten students performed better than half-day kindergarten students in the second grade. Beers said it's the results after second grade that he's interested in.

"Calling it longitudinal is over the top," Beers said of the study. "Studies do show remnant effects of full-day kindergarten by the second grade. Few show effects by the third grade. Most don't show any effect by junior high and high school."

The Legislature allocated $22 million for full-day kindergarten during the 2005 session, about $17 million of which funds low-income Clark County schools. The district has about 11,000 students enrolled in full-day kindergarten. State superintendents are asking for -- but do not expect to have fully-funded -- nearly $158 million to expand full-day kindergarten, of which about $110 million would go to Clark County.

The RAND Corporation concluded in a 1998-99 study that full-day kindergarten may not help student achievement in the long term.

And the Long Beach Unified School District in California performed a study in 2000 examining dozens of full-day kindergarten studies. It found that advantages of full-day kindergarten are not conclusive. But the study stated there were no negative findings.

The U.S. Department of Education's National Center for Education Statistics has concluded that full-day kindergarten was beneficial to students in first grade, although it showed no positive effects after the end of third grade.

The district's studies referenced several national studies lauding the benefits of full-day kindergarten, even some that concluded it is beneficial for students well past the third grade.

Carson City conservative activist Chuck Muth said the district's study should not be given any credibility.

"No one should trust anything in that report," Muth said, noting the school district got the results it wanted to get. "They are a biased source."

Alrene Lewis, the district's director of the Department of Research and School Improvement, said the study is ongoing, and students will be tested again in the third grade.

She said the study is credible and the district will be sending it to professors from universities nationwide.

"We are so confident in the results that we're going to send it out to be verified outside of the district," she said.

Review-Journal writer Lawrence Mower contributed to this report.