For over 30 years, contention has surrounded the issue of preschool children's readiness for formal schooling. On one hand, the non-standardised skills-based checklists utilised by many preschool teachers to infer children's levels of readiness for year one seem ineffectual as the debate regarding children's school readiness persists. On the other, a Piagetian view regards the use of particular operatory structures indicative of the concrete operational stage of cognitive development are necessary for a child to achieve successful learning outcomes in the new formal education environment of primary school. This paper reports on the findings from an empirical research project in which forty-two preschool children were administered a Piagetian conservation of number task as outlined in the work of Piaget and Szeminska The child's conception of number (1941/1952) using the Genevan m‚thode clinique, and were simultaneously assessed by their teachers using the preschool's Key Indicators of Readiness for Year One (KIRYO) Checklist. In addition to the routine checklisting practice, participants then judged qualitatively on their preschool and early year one performance by their respective teachers. Rasch analysis of children's performances on each of the above-mentioned indicators using the Partial Credit Model indicated that success on the conservation task was more closely aligned with children's success in year one than was mastery of the KIRYO Checklist indicators. The implications for professional discussion and decision-making, as well as insights for teachers of early childhood sectors are canvassed.