This project considers the common practice of using a single measure of school value-added as a sufficient characterization of the effectiveness of public high schools. By making the definition of a school's "causal effect" more explicit--and then modifying that definition--the author explores whether a single value-added measure is suitable for policy purposes. The author uses student-level administrative data provided by four large California school districts. Preliminary results indicate that the full specification of the school causal effect does change the rankings of schools within these four districts. If one believes that each of these definitions captures an element of the public high school's responsibility, one single measure of school value added may be insufficient for accountability policy purposes. (Contains 5 figures and 6 endnotes.)