expressions. For example, French Upper Volta and the Republic of Upper Volta were relabeled Burkina Faso. In the end, 292 specific geographic regions (first bullet above) and 236 aggregate geographic regions (second bullet) were identified. The 292 specific regions include the 226 countries and territories in the master list and 66 other single locations that can be assigned to one of the 226 in the master list (see appendix 2).<sup>15</sup> The 236 aggregate geographic regions pose larger problems. The migrants originating from a given aggregate geographic area need to be allocated to the individual countries that comprise that area. This is one of the greatest difficulties in this project, and resolving it is one of the main contributions of this work. Several propensity measures were developed depending on the quality of the data. They are based either on a destination country‘s propensity to accept migrants from a particular origin or on origin countries‘ propensity to send migrants abroad. These propensity shares are then calculated, and the resulting number of migrants are assigned, in order of quality, to specific origin countries in the master list.

Finally, the miscellaneous categories also needed to be dealt with consistently to enable meaningful comparisons between country pairs. There is often a high number of nonresponses to the question about place of birth for foreign-born residents (Bilsborrow and others 1997, p. 60). As a result, some censuses report large number of people with unknown place of birth. All these individuals are assumed to be natives in the analysis since it is unclear as to whether these persons refer to the domestically born or the foreign born. These entries are therefore deleted from the matrices. In other cases, calculations were made to check whether these totals contributed to the foreign born in each census. In most circumstances they did not, and so they were dropped. In cases when these totals did refer to migrants, they were treated as an appropriate aggregate category to be assigned later, as detailed below. Finally, all categories referring to the ―stateless‖<sup>16</sup> were dropped because despite their importance as a minority group in global migrant patterns, there is no way to meaningfully assign them to an origin.

# _Disaggregation of Aggregate Categories_

The disaggregation of the 236 origin regions identified in the censuses is one of the key steps in creating the bilateral migration matrix. Three propensity equations are used to allocate migrants to one of the 226 countries in the master list. Each measure varies in quality depending on the availability of underlying data. The preferred option is to use migration data from the destination country for the relevant year. If this option is not available, information from the destination country for other years is used. Should 15 For example, the Vatican is assigned to Italy, Wake Island to the United States, and Labuan to Malaysia.

16 Some estimates put the number of stateless people (those lacking any citizenship) as high as 11 million, although many of these people will not be captured in censuses. The stateless represent an important category of migrants; for more information, see www.unhcr.org/pages/49c3646c155.html.

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