Estimating the Number and Characteristics of Illegal Migrants via the Center Sampling Technique: Methodology and Application to Italian Data

• Activity Number: 67
• Type: Topic Contributed
• Date/Time: Sunday, July 29, 2012 : 4:00 PM to 5:50 PM
• Sponsor: Social Statistics Section
• Author(s): Gianluca Baio*+ and Gian Carlo Blangiardo and Marta Blangiardo
• Companies: University College London and University of Milano-Bicocca and Imperial College London
• Keywords: Unauthorised migration ; Sampling ; Correction of bias

Abstract

We address the problem of performing a survey of a group of individuals present in a given population, when information about the complete list of the members is missing or partially unknown. This problem is particularly relevant in migration analysis, where many of the individuals are possibly unauthorised migrants and therefore not formally registered or accounted for in official statistics. The sampling procedure consists in gathering additional information from a subset of individuals, which is then used to build suitable weights to re-proportion the sample. The bias introduced by the sampling procedure can be then corrected. We assume that the experimenter has information about a number of "aggregation centres" that are regularly visited by the migrants and where the survey can be conducted. If the experimenter can estimate the relative importance of each centre (possibly with re