New family and demographic behaviours are leading to greater individual and social mobility, making it more difficult to define and observe actual family and housing situations. Simultaneously, big statistical data, i.e. data from administrative files covering the whole population, are now becoming available to the research community. The project aims to extend our knowledge of complex and hard-to-measure situations, using several data sources including big data, and to assess the strengths and weaknesses of several data sources that will be disseminated in 2016 by the French National Institute of Statistics and Economic Studies (INSEE). Data necessary for complex demographic studies, such as the French Demographic Panel based on censuses and civil registration, tax data and family allowance data, are now becoming widely available. So far, they have been rarely used for research purposes in demography. Therefore, we propose, in a first and crucial stage, to assess and document for general use the big data sources recently made available for research, in collaboration with INSEE. This collaboration is key to the constitution of reliable and well documented data sources. The knowledge from experts from several backgrounds and institutions will be essential to fully validate and test these data sources for various uses. In this step, we will check the consistency of population estimates based on censuses, surveys and administrative data, in terms of omissions and double counts, and the impact of discrepancies on the estimation of family situations and behaviours. Two research questions, which are normally difficult to evaluate with standard surveys, will then be addressed, making use of diverse methods and sources: administrative data, censuses, population surveys and qualitative data from semi-structured in-depth interviews. . First, intimate relationships at young adult ages are known for their volatility, and are therefore hard to study with standard survey data. The new data sources will make it possible to look at forms of partnership and union stability in relation to income, education, occupation and labour market integration. This will vastly increase our knowledge of the dynamics of early adulthood and will further our understanding of new forms of partnership. Second, we will look more closely at the situation of children whose parents are separated, and who are a major source of double counting in surveys and censuses. New data sources will provide a more accurate picture of the family situation of children, including those in complex living arrangements, in relation to their standards of living and poverty risk. Administrative data are very useful for studying transitions, while retrospective surveys are often complicated by recall bias and panel studies are weakened by attrition. This project will be placed in a national and international perspective. It will provide an opportunity to combine the strengths of national institutions, while creating links with institutions abroad involved in the analysis of big administrative and census data. We will benefit from their experience and interact with big data networks to improve the quality and efficiency of our assessments and studies. By making information, documentation and code for data use available on a website, this project will have a significant impact on the scientific community. It will also contribute to the enhancement of data quality and access. The publication of methodological and applied articles in internationally reputed journals will promote the dissemination of the project’s progress and findings. Members of the project come from the French Institute for Demographic Studies (INED), INSEE, and the universities of Paris 1 Panthéon-Sorbonne, Paris Descartes, Lyons, and Strasbourg. A major output of the project will be to encourage and facilitate the use of big statistical data amongst scholars working in the humanities and social sciences.