First, because it is based on random assignment of students, it is free of key sources of bias (see Note 1). (carnegieknowledgenetwork.org)
It may be plausible to assume that the double random assignment of students and teachers applied also to specific classrooms (12, 13). (pnas.org)
Moreover, because random assignment of students to teachers occurred within schools, this effect can not be attributed to the overall effectiveness of the school. (carnegieknowledgenetwork.org)