United States. These studies relied on both the analysis of administrative data collected from national immigration agencies and on interviews with stateless individuals. Administrative data sources include central population registries, population censuses, vital records, migration-related databases, information on refugees and asylum seekers, and data related to enforcement of immigration laws. Table 7 details the sources used by country. As is apparent from the table, the vast majority of projects draw at least in part on central population registries (UNHCR, 2011a, 2012, 2014a, 2014b, 2015, 2016a, 2016b, 2017a), a source not available or likely to be available in the United States. Moreover, most of these studies did not make estimates of national stateless populations or (if they did) their estimates did not include unauthorized stateless populations. Table 7. Data sources examined to obtain estimates of stateless populations Country Central population Population Vital records registry** census Central migration-related Refugee & Asylum database seeker Australia* X Austria X Belgium X Estonia X Finland X Iceland X X X Naturalizations X X X X X X X X X X Malta X X X X X Norway X X Sweden X X The Netherlands X The United Kingdom X Foreigners with or applying for legal residence X Japan* Lithuania Immigration enforcement (inc. detention) X X X X X X X X * Not a UNHCR “Mapping Statelessness” report ** May include information from other columns Sources: Abe (2010), UNHCR (2011a, 2012, 2014a, b, c, 2015b, 2016b,c, 2017a), UNHCR and Asylum Aid (2011), and Foster, McAdam, and Wadley (2017). Reports from these projects suggest three key difficulties with administrative data. First, administrative data often records the nationality of individuals as “unknown.” In some cases, individuals with “unknown” nationalities may be stateless, but it is not known how often this is the case. As a consequence, estimates of stateless persons based on registry data may not be accurate. Moreover, in many countries such as Belgium and The Netherlands, registration is carried out at the municipal level, and the determination of statelessness often varies across municipalities (UNHCR 2011a, 2012). Second, official records such as registries do not generally include migrants without lawful status. Often, registries are restricted to legal residents. Third, stateless persons may seek to avoid official detection because of their precarious situation, which may lead to undercount. Six countries also interviewed stateless individuals in order to gain qualitative understanding of their experiences (Austria: UNHCR 2017a; Belgium: UNHCR 2012; Estonia: UNHCR 2016, Lithuania: UNHCR 2016a; Malta: UNHCR 2014c; Netherlands: UNHCR 2011a; United Kingdom: 67