It concluded that keeping online pupils focused on their work was the biggest problem faced by online charter schools, and that in mathematics the difference in attainment between online pupils and their conventionally educated peers equated to the cyber pupils missing a whole academic year in school. State-specific structure and regulations State laws follow varied sets of key organizing principles based on the Citizens League's recommendations for Minnesota, American Federation of Teachers guidelines, or federal charter-school legislation (U.S. Department of Education). Principles govern sponsorship, number of schools, regulatory waivers, degree of fiscal/legal autonomy, and performance expectations. Center for Education Reform ranking Current laws have been characterized as either "strong" or "weak." "Strong-law" states mandate considerable autonomy from local labor-management agreements and bureaucracy, allow a significant number of charter schools to be authorized by multiple charter-granting agencies, and allocate a level of funding consistent with the statewide per pupil average. According to the Center for Education Reform, a pro-charter group, in 2015 the District of Columbia, Michigan, Indiana, Minnesota, and Arizona had the "strongest" laws in the nation. Maryland, Virginia and Kansas are home to the nation's "weakest" laws, according to the same ranking. National evaluations Multiple researchers and organizations have examined educational outcomes for students who attend charter schools. In general, urban charter schools may appear to be a good alternative to traditional urban schools for urban minority students in poor neighborhoods, if one looks strictly at test scores, but students in suburban charter schools do no better than those in traditional suburban schools serving a mostly middle-class white population. Center for Research on Education Outcomes CREDO studies charter schools and has completed two national reports for 2009 and 2013. The report is the first detailed national assessment of charter schools. The reports analyze the impact of charter schools in 26 states and find a steady improvement in charter school quality since 2009. The authors state, "On average, students attending charter schools have eight additional days of learning in reading and the same days of learning in math per year compared to their peers in traditional public schools." Charter schools also have varying impacts on different demographic groups. Black students in charters get an extra 7 days of learning in reading. For low-income charter school students the advantage is 14 days of extra learning in reading and 22 days in math. English Language Learner students in charter schools see a