A Princeton-based research firm's analysis of the KIPP national network of charter schools found KIPP middle-school students made substantial gains in core subjects over a three-year span.

Mathematica Policy Research was scheduled to release a report Wednesday based on what it called its "most rigorous large-scale evaluation of KIPP charter schools."

KIPP, which has four charters in Philadelphia, contracted with the nonpartisan research firm to perform several long-term studies. The network of 125 schools in 20 states and the District of Columbia focuses on a rigorous college-prep education for low-income students.

Two city charters - KIPP Philadelphia in North Philadelphia and KIPP West Philadelphia Preparatory - were among the 43 middle schools in the study.

"I think this is one more affirmation that what we are doing in Philadelphia and nationally is working," Marc Mannella, chief executive officer of KIPP Philadelphia, said Tuesday.

The nonprofit Knowledge Is Power Program (KIPP) is known for a longer school day, with more instructional time than traditional public schools and many other charters.

In Philadelphia, KIPP students are in school from 7:30 a.m. to 5 p.m. weekdays. They attend programs on alternate Saturdays and get three weeks of lessons in the summer.

In a briefing Tuesday, Mathematica researchers said they found that three years after enrolling at KIPP middle schools, which span grades five through eight, the average student made greater advances in reading, math, social studies, and science than his or her counterpart attending a local public school.

Based on scores from state tests, Mathematica found that after three years at a KIPP middle school, the average student had moved from the 44th to the 58th percentile in math and from the 46th to 55th percentile in reading.

And after three to four years, the student had moved from the 36th percentile to the 49th percentile in science and from the 39th percentile to the 49th percentile in social studies.

The report does not give results for individual charter schools.

"KIPP is making important strides to close achievement gaps for disadvantaged students," said Philip Gleason, a senior fellow at Mathematica who directed the study. "Findings from this large and comprehensive evaluation show that KIPP schools lead to educationally meaningful increases in student achievement, not just in basic reading and math but in a broader set of subjects, including science and social studies."

For the study, which involved 30,000 students, researchers used two methods to evaluate KIPP's performance. They matched KIPP students with counterparts in their local schools based on income, race, and achievement level, and then compared their academic performance over time.

And at 13 KIPP schools where students had participated in admission lotteries, researchers compared the students who were selected for admission with those who did not win seats in the lottery. This approach ensures the positive results were not based on parental involvement and student motivation, researchers said.

Students in the lottery sample also took the national Terra Nova tests, for which they had not prepared and which had no consequences for themselves, their teachers, or their schools. The students' performance on the Terra Novas "suggest that the positive impacts of KIPP are not a result of 'teaching to the test' on state assessments," the researchers said.

Ronald Zimmer, an associate professor at Vanderbilt University who has researched charter schools for years, praised the Mathematica study.

"It looks like a high-quality study, and shows significant promise for KIPP schools and the effects they have on students," said Zimmer, who was not involved in the Mathematica research.

"It's an excellent study, and the people who are doing the work are excellent," agreed Gary Miron, a veteran charter school researcher at Western Michigan University.

But Miron, one of the authors of a controversial 2011 report on KIPP that criticized student attrition and funding, said Mathematica also should have examined what happens to students who leave KIPP because they are not succeeding and whether school districts - which must accept all students - could achieve similar results if they used KIPP's model at struggling schools.

"They are a school for college prep," Miron said. "They have a very rigorous program, but it is not a model that's going to serve everyone."

Five KIPP charters operate in Newark, N.J. KIPP Cooper Norcross Academy is planned to open in 2014 in Camden as one of the state's first privately run and publicly financed Renaissance schools.

The Norcross Foundation was created by the family of State Sen. Donald Norcross and his brother, George E. Norcross III, who is chairman of Cooper University Hospital and a managing partner of the company that owns The Inquirer.

George Norcross' daughter, Alessandra, a director of the parent company of The Inquirer, is an officer of the foundation and a board member of KIPP Cooper Norcross Academy.

Contact Martha Woodall at 215-854-2789 or martha.woodall@phillynews.com