but most are granted for 3–5 years. Operational autonomy Often, charters create unique school cultures to tailor to students, particularly minority students in urban school districts, whose school performance is affected by social phenomena including stereotype threat, acting white, non-dominant cultural capital, and a "code of the street". Most teachers, by a 68 percent to 21 percent margin, say schools would be better for students if principals and teachers had more control and flexibility about work rules and school duties. Accountability for student achievement Charter schools are accountable for student achievement to their sponsor — a local school board, state education agency, university, or other entity — for producing positive academic results and adhere to the charter contract. While this accountability is one of the key arguments in favor of charters, evidence gathered by the United States Department of Education suggests that charter schools may not, in practice, be held to higher standards of accountability than traditional public schools. Typically, these schools are allowed to remain open, perhaps with new leadership or restructuring, or perhaps with no change at all. Charter school proponents assert that charter schools are not given the opportunities to restructure often and are simply closed down when students perform poorly on these assessments. , 12.5% of the over 5000 charter schools founded in the United States had closed for reasons including academic, financial, and managerial problems, and occasionally consolidation or district interference. A 2013 Study by the Center for Research on Education Outcomes (CREDO) at Stanford University institute linked overall improvement of the charter school sector to charter school closures, suggesting that charter schools as a whole are not getting better, but the closure of bad schools is improving the system as a whole. Many charter schools are created with the original intent of providing a unique and innovative educational experience to its students. However, charter schools are still held accountable for test scores, state mandates, and other traditional requirements that often have the effect of turning the charter school into a similar model and design as the public schools. Although the U.S. Department of Education's findings agree with those of the National Education Association (NEA), their study points out the limitations of such studies and the inability to hold constant other important factors, and notes that "study design does not allow us to determine whether or not traditional public schools are more effective than charter