This book draws on the analysis of national curriculum test data from more than 600 children of 10-11 and 13-14 years of age, as well as in-depth interviews with 250 of these students, as they attempt to solve test problems, in order to explore the nature of the difficulties children experience with realistic items. It is shown, by comparing test and interview data, that many children, as a consequence of their confusion over the requirements of realistic' test items, fail to demonstrate on tests the mathematical knowledge and understanding that they actually possess. This book also explores whether the problem of invalid measurement is equally spread across children from different social backgrounds and across the sexes. Chapters include: (1) the research--origins, methods, issues; (2) children and realistic test items--reflecting on previous studies; (3) realistic items--two children, two cultural competencies; (4) class, sex, contextualization and performance--a quantitative analysis at Key Stage 2; (5) realistic items, class and sex--two examples from Key Stage 2; (6) class, sex, selection for tiers, and performance--a quantitative analysis at Key Stage 3; (7) children's answers to items and explicitness--examples from Key Stage 2 and Key Stage 3; and (8) constructing the right goal--a comparative analysis of two Key Stage 3 items. (Contains 147 references.) (ASK)