The Census of Population and Housing is one of the most important surveys carried out by ISTAT. It is conducted every ten years from 1861, and the main objectives are: the count of the whole population and the recognition of its structural characteristics; updating and revision of civil registers; the definition of the legal population for juridical and electoral purposes; the collection of information about the number and structural characteristics of houses and buildings. The Census collects information about demographic and family structure of the population, the types of their households, their level of education, their employment status, and other informations on residents population. In 2011, for the first time, some information of socio-economic character were measured on a sample basis through the use of two types of questionnaire: one in a reduced form, with a few questions, including indispensable information for the production of the data required by the European Union with an high spatial detail, and one in complete form. In particular, Istat provides a 1% sample data (594,247 cases) released in two separate datasets: the first file (individui) refers to persons usually resident in private households and in Institutional households and the second one (alloggi) refers to living quarters. In urban areas with at least 20,000 inhabitants a sample was selected by a simple random sampling without replacement procedure of one third of the families. A complete version (long form) of the questionnaire has been sent to the sample, while a short version the questionnaire has been sent to all other inhabitants. web-based self-administered questionnaire (CAWI)