Young children\u27s concept of what units can be counted—for example, individuals, classes and kinds—changes with age. It is important to understand how young children master the concept of counting varied units given that mastering alternative units is a basic skill in understanding our base system of counting (e.g., understanding that ten tens equal 100). An ability to enumerate classes would appear to be one precursor to being able to adopt alternative units of counting and to attaining later math concepts (i.e., base systems). The facilitation of 4-year-olds ability to enumerate classes was examined for three variables: (1) alternative class labels, (2) salient unifiers (containment) and (3) set size. In Experiments 1 and 2, four-year-olds\u27 ability to enumerate classes with different labels was examined. In Experiment 3, four-year-olds\u27 ability to enumerate classes with contained sets was examined. Lastly, in Experiment 4, four-year-olds\u27 ability to enumerate classes with large size sets was examined. Overall, about 50% the subjects could not quantify classes: most children\u27s responses were bimodal. A bias to process discrete physical entities rather than labels or perceptual properties appears to account for the children\u27s performance
