A goal of NAEP has been to provide information to the public about what students in United States schools know and can do in various subject areas. While the NAEP scales in each subject area provide information about the distributions of scale scores for various student groups of interest, they do not directly provide information about the meaning of specific points on the scale.
Traditionally, meaning has been attached to educational scales by norm-referencing—that is, by comparing students at a particular scale level to other students. However, NAEP uses scale anchors and achievement levels to describe selected points on the scale in terms of the types of skills that are likely to be exhibited at those levels. Scale anchoring is accomplished through the process of item mapping. In item mapping, individual NAEP assessment items are mapped to a point on its corresponding scale, so that the content of each item provides information about what students at each score level can do in a probabilistic sense. Achievement levels have been defined for the NAEP economics, geography, mathematics, reading, science, and U.S. history composite scales and for unidimensional subscales in civics and writing. The achievement levels were set for mathematics in 1990, reading in 1992, geography and U.S. history in 1994, science in 1996, civics and writing in 1998, and economics in 2006.