School districts are a focus of education reform efforts in the United States, but there is very little existing research about how important they are to student achievement. We fill this gap in the literature using 10 years of student-level, statewide data on fourth- and fifth-grade students in Florida and North Carolina. A variance decomposition analysis based on hierarchical linear models indicates that districts account for only a small share (1 to 2 percent) of the total variation in student achievement. Nevertheless, the differences between lower- and higher-performing districts are large enough to be of practical and policy significance, with a one standard deviation difference in district effectiveness corresponding to about 0.11 standard deviations in student achievement (about nine weeks of schooling). District performance is generally stable over time, but there are examples of districts that have shown significant increases or decreases in performance.

One of the major story lines of the growth of civilization is the advance of the experiment. From the food we eat to the diseases we conquer to our understanding of how we think and behave, we have profited enormously from an approach that marries our models of the world with tests of their validity through systematic variation to determine cause and effect. These same tools of thought and action are no less critical to the advance of education than to medicine, agriculture, psychology, or transportation. There has been impressive growth in the use of experimental methods in education over the past decade. As a result, much more is known today about what works and what does not. Future progress will be enhanced by research that better explicates process: linking trials to administrative data; serving the interest of local and state officials in examining the impact of their policies and practices; better preservice training for education practitioners in the logic and value of rigorous research; and more generous federal funding for the research enterprise.