What and how you ask determines what you get

Abstract This article has a methodological focus, looking at how to study early numeracy when the intention is not to find out whether a child knows something about numbers or not (usually measured in quantitative terms “knowing or not knowing”) but to, in an ecologically valid way, identify the different ways in which (i.e., how ) children understand and use numbers. We present the theoretical basis for five design principles on which play-based interviews were developed and conducted with children 1–3 years old. We argue, and illustrate with empirical examples, that the tool developed is an ecologically valid way to study early numeracy involving very young children. We also argue that individual children cannot be identified with certainty as “cardinality knowers” as their understanding and use of numbers is situated. By identifying differences in how children understand and use numbers, it is also possible to observe changes over time.