Reports on three studies which investigated the development of the concept of horizontality in subjects from nursey school through college age. Normative data collected allowed for evaluation of Piaget's stages and for differences due to age, sex, and apparatus shape and orientation. (ED)

Three samples of 8- to 16-year olds were assessed three times at yearly intervals on eight water-level items. Within-child change over age was viewed as stochastic process of the child changing or remaining in one of three latent strategy states. Although there was improvement in task performance over age, the general finding was that strategy…