for the same destination country would yield inconsistent data. Priority is given to the foreign-born definition, and these data are always used if at least three censuses using that definition and with detailed bilateral information are available for a particular country. However, only nationality data are available for many destination countries. For countries such as Japan that rarely offer citizenship to foreigners, this does not pose much of a problem since foreign-born and nationality data will be very similar. For other destination countries, including data based on the nationality concept will lead to disparities. When fewer than three foreign born data sources are available and the nationality data are of superior quality, the nationality definition is chosen (see appendix 1). Where fewer than three data points by either definition are available, several assumptions are made to fill the missing data.

# _Missing Censuses and Census Data_

The final hurdle in constructing the global migration matrices is dealing with omitted data. No census round is truly complete since no round has ever included every country in existence at the time. Censuses are expensive because of their universal coverage and labor intensity. For those reasons, many countries have started to conduct censuses only recently (Bhutan began in 2005). Censuses can also be abandoned because of civil unrest or military conflict. They can also be politicized, because they can be used to estimate the size of a particular ethnic group. In other words, data may simply never be released even if they are collected. Nor is there any guarantee that a question on nationality or country of birth will even be included in the census questionnaire. Many countries in Central Asia, as well as Fiji, Sri Lanka, and Tonga, have in some years included questions on ethnicity instead, which is useless for identifying migrants. For all these reasons, inevitable gaps in the data emerge (see table 2).

Three conventions are adopted for constructing missing data. The one that is ultimately used depends on how many data are missing and for which decades these data are missing relative to the decades for which data are available.

MISSING IN-BETWEEN DECADES. Where data are missing for a particular decade but are available for the decade before and after, a linear trend is assumed between the earlier and later bilateral data. In total, 86 country-years of data were interpolated using this method.

MISSING BEGINNING OR END DECADES. Where the data are missing at the beginning or the end of the time period, the destination country is assumed to have the same bilateral migrant composition as in the decade closest to the missing period. The bilateral shares from the closest decade for which data are available are applied to the destination country‘s total number of migrants for the missing decade. The information comes from one of two sources. In some cases, the census provides the total number of migrants without any bilateral information. If these data are not available, the total from the closest decade is taken and adjusted for growth in migration. The growth rates are taken from Trends in International Migrant Stock, which details total migrant stocks for 13