Are teachers the most important factor affecting student achievement?
This has become the default first sentence of many speeches and reports on teacher quality. Recently, it’s become common to clarify that teachers are the most important “school-based” factor in learning—a critical qualification, given that factors external to schools exert more influence overall on student achievement than any factors inside the school.
A famous 1966 study by James Coleman found that background characteristics such as race, parental achievement levels, and family income swamped most other factors studied as determinants of student test scores. Decades of research have confirmed this study’s general findings, with a 1999 paper estimating that 60 percent of variation in student achievement was attributable to such background characteristics. 
Researchers have been unable to link a significant share of the variation in student achievement—as much as 25 percent—to any particular input. Of the remaining share, attributable to what happens within school, researchers have linked most of that variation to teachers.
It is difficult to cite an exact figure on what percent of the variation in achievement observed is attributable to differences in teacher effectiveness. Three economists in 1998 estimated that at least 7.5 percent of the variation in student achievement resulted directly from teacher quality and added that the actual number could be as high as 20 percent.
Researchers have found that school-based factors, including teaching, are more influential in math than in reading. A 1999 paper puts all in-school factors, including school-, teacher-, and class-level factors, at approximately 21 percent of the variation in 10th grade mathematics achievement. It further estimated that 8.5 percent was directly due to teacher effectiveness.
Some researchers warn that other important factors that potentially affect achievement— such as the effect of principals and other administrators, and the interaction of teachers with the curriculum—have not been as carefully studied as teacher quality.
It can be said:
Research has shown that the variation in student achievement is predominantly a product of individual and family background characteristics. Of the school factors that have been isolated for study, teachers are probably the most important determinants of how students will perform on standardized tests.
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