Database. Where both censuses and population registers are available, censuses receive priority.

Censuses, generally conducted decennially, are retrospective tools for surveying an entire population (or in some cases, a representative sample) at a single point in time. In addition to their universal coverage, their greatest strength is the inclusion of questions on place of birth and nationality. Censuses also typically aim to enumerate the resident population, whether documented or undocumented (Bilsborrow and others 1997). So although some migrants have a strong incentive to provide false information to enumerators, many undocumented migrants will be captured in these matrices.<sup>7</sup> The size and scope of the census questionnaires vary enormously, both over time and in different destination countries. And there is potential variation in the quality of censuses both across countries and over time. Richer countries have many resources at their disposal to design questionnaires, train interviewers, employ statisticians, and disseminate results. Researchers have little choice but to accept the data at face value. However, where the underlying census is clearly substandard (when there are errors that are obviously not coding errors or not easily corrected), these data are omitted from the analysis.

Popular in many parts of Europe, population registers are continuous reporting systems providing up-to-date demographic and socioeconomic information for everyone surveyed. Typically, registers have evolved over time (from parish records, for example). They were never developed specifically to record international migration information, and they vary considerably across countries. For example, the laws under which individuals are classified as migrants and the conditions under which they are inscribed or deregistered differ greatly (Bilsborrow and others 1997).

# _The Raw Data_

The Global Migration Database is a vast collection of destination country data sources detailing migrant stocks from numerous origin countries and regions (United Nations [2008]). Compiling and maintaining the underlying primary sources require herculean efforts to scour the key census collections of the world and enter the data manually. In total, the database comprises records from some 3,500 separate censuses from more than 230 migrant destination countries and territories, by sex and age. Destination countries make numerous revisions between census waves,<sup>8</sup> and the database incorporates as many of these revised figures as possible.<sup>9</sup> The starting point is to choose the most relevant source for each destination country from each completed census round.<sup>10</sup> Priority is given to data that are superior 7 The extent to which illegal migration is captured remains unknown.

8 Census results are also often released in waves, typically beginning with preliminary estimates and following with incremental releases of more detailed data.

9 The raw data are available at http://esa.un.org/unmigration.

10 Bhutan, Colombia, and El Salvador did not conduct censuses during the 2000 round; the relevant censuses for 2005 or 2007 are included instead. Similarly, for seven countries without 1960 censuses, data from the 1950 census round are included. In these 7