ABSTRACT
This chapter provides historical background on the nature of child development and discusses several general quantification approaches. The chapter explains how to apply the unit-based approach to arrive at the D-score scale. The standard set in the Dutch Development Instrument consists of relatively easy milestones that 90 per cent of the children can pass at the scheduled age. This set is designed to have maximal sensitivity for picking up delays in development. Most measurement models describe the probability of passing an item as a function of the difference between the person’s ability and the item’s difficulty. A person with low ability will almost inevitably fail a heavy item, whereas a highly able person will almost surely pass an easy item. The logistic function is the dominant choice in item response theory, but it is instructive to study some other mappings. The item response function maps success probability against ability.
