Most people recognise that a low level of literacy skill can make it difficult to function effectively in adult life, but it is often assumed that numeracy (i.e. being able to deal competently with numbers, tables and graphs) is less important than literacy. This study was designed to test whether this is in fact the case. The study draws on data from two major longitudinal studies of the British population. Longitudinal studies are designed to follow the same group of people over their lifetime. They can identify patterns of change and can show how particular characteristics and experiences affect what happens to people over the period in question.