The purpose of using normative information is to understand a student’s test performance in the context of how well his or her peers of similar age or years of schooling are performing. Ability tests measure reasoning skills that are developed through experience. We would not directly compare a 5-year-old’s performance on the CogAT tasks to a 10-year-old’s because the 10-year-old has had more opportunities to develop reasoning skills (not to mention the many other maturational and developmental processes at work).
This is why we use age groups in norming tests of cognitive abilities—so that students of similar age and experience are compared rather than students with a wider range of school and overall experience. We are seeking to control for differences in education, experience, and maturation in order to compare “like to like.” This is also why local norms are especially helpful, because they compare students to peers that are not only of a similar age, but also from the same school district with more similar resources and experiences. This allows a better interpretation of how well a student reasons with the CogAT tasks compared to similar students.