Most of New York City's 99 charter schools, which enroll 30,000 students, have gotten superior results on state tests. It is important to understand why many of them perform so well, since Mayor Bloomberghas promised to double the number of charters over the next four years.Last fall, a report by economist Caroline Hoxby of Stanford University hailed the city's charter schools and suggested that any student who attended a charter school for nine years would be almost as well educated as a student in well-heeled Scarsdale. A new study by economistMargaret Raymond of Stanford has confirmed that many of the city's charter schools get higher test scores. Raymond found that 51% of New York City charters produced significant gains in math, but only 29% did so in reading.