43-day learning advantage over traditional public school students in reading and an extra 36 days advantage in math. Charter schools showed a significantly greater variation in quality between states and within states. For example, Arizona charter school students had a 29-day disadvantage in math compared to public school students but charter school students in D.C. had a 105-day advantage over their peers in public schools. While the obvious solution to the widely varying quality of charter schools would be to close those that perform below the level of public schools, this is hard to accomplish in practice as even a poor school has its supporters. Criticism and debate Stanford economist Caroline Hoxby criticized the study, resulting in a written debate with the authors. She originally argued the study "contains a serious statistical mistake that causes a negative bias in its estimate of how charter schools affect achievement," but after CREDO countered the remarks, saying Hoxby's "memo is riddled with serious errors" Hoxby revised her original criticism. The debate ended with a written "Finale" by CREDO that aimed to rebut both Hoxby's original and revised criticism. The National Education Policy Center has criticized the methods that CREDO has used in its studies. They criticized the CREDO studies for "over-interpreting small effect sizes; failing to justify the statistical assumptions underlying the group comparisons made; not taking into account or acknowledging the large body of charter school research beyond CREDO's own work; ignoring the limitations inherent in the research approach they have taken, or at least failing to clearly communicate limitations to readers." National Bureau of Economic Research study In 2004, the National Bureau of Economic Research found data that suggested charter schools increase competition in a given jurisdiction, thus improving the quality of traditional public schools (noncharters) in the area. Using end-of-year test scores for grades three through eight from North Carolina's state testing program, researchers found that charter school competition raised the composite test scores in district schools, even though the students leaving district schools for the charters tended to have above average test scores. The introduction of charter schools in the state caused an approximate one percent increase in the score, which constitutes about one quarter of the average yearly growth. The gain was roughly two to five times greater than the gain from decreasing the student-faculty ratio by 1. This research could partially explain how other studies have found