Collecting data from hard-to-reach populations is a key challenge for research on poverty and other forms of extreme disadvantage. With data from the Boston Reentry Study (BRS), we document the extreme marginality of released prisoners and the related difficulties of study retention and analysis. Analysis of the BRS data yields three findings. First, released prisoners show high levels of "contact insecurity," correlated with social insecurity, in which residential addresses and contact information change frequently. Second, strategies for data collection are available to sustain very high rates of study participation. Third, survey nonresponse in highly marginal populations is strongly nonignorable, closely related to social and economic vulnerability. The BRS response rate of 94% over a 1-y follow-up period allows analysis of hypothetically high nonresponse rates. In this setting, nonresponse attenuates regression estimates in analyses of housing insecurity, drug use, and unemployment. These results suggest that in the analysis of very poor and disadvantaged populations, methods that maximize study participation reduce bias and yield data that can usefully supplement large-scale household or administrative data collections. Under current conditions of historically high imprisonment rates, incarceration has become a regular part of the hardship associated with American poverty (4, 5). Men and women who go to prison present a range of co-occurring disadvantages and vulnerabilities that make them difficult to contact or unwilling to participate in research studies. Involvement in crime and the criminal justice system also makes study subjects elusive for researchers. As a result, follow-up studies of the formerly incarcerated have suffered from high rates of attrition that are likely correlated with outcomes of key interest. The incarcerated population has been called "invisible men" because of their underenumeration in social survey estimates of socioeconomic well-being (6). Because of their deep marginality in social and economic life, invisibility in social science data collection also extends to those who have moved from incarceration to communities.We explore survey nonresponse in a highly disadvantaged population using data from the Boston Reentry Study (BRS), a small longitudinal survey of newly released prisoners. Earlier longitudinal studies of released prisoners suffered from 30% to 60% attrition over a 1-to 2-y follow-up period (7,8). In contrast, the BRS followed Massachusetts state prisoners through the first year of prison release, achieving a response rate of 94%.The unusually high response rate allows us to explore three main challenges to research and data collection in highly disadvantaged populations. First, we document extreme social insecurity in the BRS sample that is closely associated with a high level of contact insecurity-the frequent changing of residential addresses and phone numbers that makes nonresponse more likely. Second, we describe a variety of strategies for data collection tha...