The arbitrary nature of growth trends in cognitive variables is illustrated. Two metrics, grade equivalent and item-response theory representations, both of which preserve the order of performance levels in test data, produced different pictures of cognitive growth, and differences were seen to arise from differences in the scaling models. (SLD)

It has long been a part of psychometric lore that the variance of children's scores on cognitive tests increases with age. This "increasing-variance" phenomenon was first observed on A. Binet's intelligence measures in the early 1900s. An important detail in this matter is the fact that developmental scales based on age or grade have…