It is commonplace to see information about student achievement presented by comparing test scores to a defined performance standard. We read, for example, that only 31% of fourth graders performed at or above a “proficient” level in reading on the National Assessment of Educational Progress (NAEP), that 86% of school children tested in Nebraska are at the “proficient” level in writing, that more than half of all twelfth graders lack even a “basic” understanding of United States history, or that 42% of California students scored at or above “proficient” on the California Standards Tests in English Language Arts. Few stop to ask what labels like basic or proficient actually mean, or how the lines dividing one performance level from another are drawn. Moreover, standards are used not only to describe educational achievement, but also to set requirements and expectations. State-level school accountability systems mandated under the No Child Left Behind Act of 2001 (NCLB) require that annual measurable objectives in both reading and mathematics be formulated as the percentage of all students required to reach the state’s “proficient” level for each school as well as for specified subgroups within a school.1