NEW YORK, NY September 22, 2009 —A new study by Stanford researchers finds students who attend New York City charter schools for both elementary and middle grades are closing the achievement gap with suburban students.

On average, a student who attended a charter for grades K through 8 would close 86 percent of the so-called "Scarsdale-Harlem" achievement gap on state math exams. The gap narrowed a little less in English.

Caroline Hoxby, who directed the study, says it could be that charter school students spend a lot more time in school.

HOXBY: The average charter school in NYC has two and a half more weeks in a school year, two more hours in a school day, than the traditional public schools do. It's a very long school year.

The study is considered significant because the charters use random lotteries, and it compared students who got into the charters with those who didn't - meaning they were demographically similar and their families were equally motivated.

The study was conducted by a Stanford-based researcher for the New York City Charter Schools Evaluation project, and looked at 43 schools. Researchers attributed the success of charters partly to longer school years, and to paying teachers based on performance.