For every subject assessed, NAEP reports on how well students in different demographic groups (e.g., race, gender, and region) perform on the assessment. (NAEP does NOT report individual student scores.) How does NAEP summarize what students in these groups know and can do, and make comparisons among the achievement of these groups of students?
In science, NAEP creates a scale ranging from 0-300, based on statistical procedures called Item Response Theory (IRT). IRT is a set of statistical procedures useful in summarizing student performance across a collection of test exercises requiring similar knowledge and skills. All NAEP subject area scales are produced using these procedures.
To give meaning to the levels of the scale, it is useful to create an "item map." An item map is a representation of the skills and abilities demonstrated by students at various levels of the NAEP science scale. The map indicates the kinds of questions students are likely to answer correctly at each level on the scale. To get a more complete sense of the science scale, explore the science item maps.