Abstract: | Survey data from cross-sectional snapshots give an incomplete picture of the social fabric. Without longitudinal information, planners, practitioners, social scientists and policy makers would be in the dark about dynamics, durations and pathways of human development. Longitudinal data collected in birth cohort studies give a better guide to long-term processes and outcomes and are more informative about the causal relations that are the drivers of disadvantage or success. The content and structure of Britain's Birth Cohort Studies are described to illustrate the challenges of creating and studying longitudinal evidence. Examples of findings and policies based on these prospective longitudinal studies are given, along with a discussion of the practical decisions that have to be confronted in undertaking them. |