So-called achievement gaps – the differences in average test performance among student subgroups, usually defined in terms of ethnicity or income – are important measures. They demonstrate persistent inequality of educational outcomes and economic opportunities between different members of our society.
So long as these gaps remain, it means that historically lower-performing subgroups (e.g., low-income students or ethnic minorities) are less likely to gain access to higher education, good jobs, and political voice. We should monitor these gaps; try to identify all the factors that affect them, for good and for ill; and endeavor to narrow them using every appropriate policy lever – both inside and outside of the educational system.
Achievement gaps have also, however, taken on a very different role over the past 10 or so years. The sizes of gaps, and extent of “gap closing," are routinely used by reporters and advocates to judge the performance of schools, school districts, and states. In addition, gaps and gap trends are employed directly in formal accountability systems (e.g., states’ school grading systems), in which they are conceptualized as performance measures.
Although simple measures of the magnitude of or changes in achievement gaps are potentially very useful in several different contexts, they are poor gauges of school performance, and shouldn’t be the basis for high-stakes rewards and punishments in any accountability system.