Difficulties with rational numbers have been explained by a natural
number bias, where concepts of natural numbers are inappropriately
applied to rational numbers. Overcoming this difficulty may require a
radical restructuring of previous knowledge. In order to capture this
development, we examined third- to fifth-grade students' understanding
of the size of rational numbers across a 1-year period. On three
occasions, students answered a test about the size of decimals and
fractions. Latent transition analysis allowed profiling five different
rational number size concepts and mapping students' developmental
trajectories during the early learning of this topic. Development from
natural number-based reasoning to mathematically correct understanding
was almost nonexistent during this time period. Instead, students appear
to move through intermediary concepts. Locating these temporary and
mediating number concepts supports the idea of a slow and gradually
developing conceptual change in rational numbers.