The production of numerical estimates is an important component of children's developing mathematical skills, and is included in the national curriculum in the UK. Previous research has suggested that estimation is affected by the way we mentally represent numbers along a 'number line'. If our mental representations correspond with actual mathematical representations then our estimations stand a better chance of being accurate. Importantly, non-linear representations might hold back the development of mathematical skills and concepts. The experimental research project will involve small teams of psychologists collecting data from children in the UK as well as from China. This will allow cross-cultural comparisons of number skills and help to understand reasons behind current differences in numerical confidence and competence between these countries.