retrospective system for surveying the entire population at a single point in time, is generally considered the most comprehensive record of the total population. Perhaps the greatest strength of census data for measuring migrant populations, besides their universal coverage, is the limited scope of the questions asked, which facilitates comparability across countries. However, censuses are infrequent and although virtually every country in the world conducts censuses, invariably they are carried out on different dates within census "rounds" that usually span a decade. The latest, 2000, round of censuses that included nearly all countries<sup>8</sup> covers censuses taken between 1995 and 2004.<sup>9</sup> Two definitions of the total (resident) population are commonly applied in censuses: de facto and de jure. The de facto population refers to all persons physically present in a country at the census moment. The de jure population refers to all those persons who either are usually resident, or who qualify as legally resident, at the census moment. They are often defined as those who have been resident in that country for a particular length of time, which can range from between 3 months to 1 year. Traditionally, censuses define migrants either as the foreign-born, or as the foreign population, although in recent years a growing recognition of the value of obtaining both sets of data has emerged.

An alternative source for calculating migrant stocks is the population register, although these are more often used to measure flows. Population registers are continuous reporting systems for recording births, deaths and changes of residence in a population and so offer data quite different from those in censuses. However, few countries maintain population registers and those that do rarely offer a comprehensive view of either country of birth or nationality. Data from registers are employed in this study only where appropriate census data were unavailable.

Of course, in an ideal world, we would like to have flow as well as stock data on migration. These can be inferred from census data through the inclusion of questions asking where people used to live in the past. Unfortunately, these data are not available for enough countries to give a broad coverage of origin and destination flows for specific time periods. A second potential source of flow data is immigration records, which provide a continuous register of all legal migrants into a country within specific periods of time and are usually published annually. Again, however, the coverage of such records is patchy and the information collected and processed highly variable, so that in the end they are not a basis for a global dataset. Even developed countries rarely maintain a record of those leaving, irrespective of whether they are natives or foreigners. Looking to a utopian future, immigration and emigration records could provide an ideal source for the study of international migration, especially if they are linked to migrants’ domestic records, as is increasingly called for by security services, but there is still a long way to go!

> 8 Those not taking part include: Afghanistan, Colombia, Peru, North Korea, Myanmar, Bhutan, Taiwan, Uzbekistan, Tajikistan, Moldova, Bosnia and Herzogovina, Western Sahara, Guinea-Bissau, Liberia, Togo, Nigeria, Chad, Cameroon, Gabon, Sudan, Ethiopia, Eritrea, Somalia, Angola, Democratic Republic of Congo, and Madagascar. (Source: United Nations, http://unstats.un.org/unsd/demographic/sources/census). 9 It must be emphasized that, although the vast majority of countries conducted censuses during this round, many have still to process and publish the data.

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