significantly to the domestic population as foreign citizens become naturalized. For our purposes, the disparity between two immigration systems will impact on the proportion of the foreign relative to the national population.

Finally, it should be noted that the way in which immigration statistics are measured has caused several discontinuities in the data, such as that resulting from the break-up of the FSU, Yugoslavia and Czechoslovakia. All of these events produced millions of additional international migrants overnight as new boundaries were drawn and new nationalities created. The data have been adjusted accordingly to include these new migrants. On the other hand, the return of Hong Kong to Chinese sovereignty in 1997 did not reduce the number of migrants: the Chinese are still regarded as “migrants” and contribute over 2 million (1 percent) to the United Nations total.

## **_The Data Collected_**

Fully aware of the challenges of data compilation, we now describe the new dataset. The aim of this study was to include as many of the world's migrants as possible, to assign them _all_ to specific countries of origin with the highest degree of accuracy and to produce as full and comparable a bilateral database of international migration stocks as is possible. In its final form, the data should be easy to interpret, suitable for a wide range of academic purposes and easy to update should further material become available. In most cases, data were recorded from their original source. Data from the latest census round were preferred, as these were considered most comparable at the global level, although given the significant lags that often exist between the timing of censuses and their publication "latest" was not always very recent. Data on both foreignborn and foreign nationals were compiled where feasible. Population registers were then drawn upon where censuses were unavailable for the 2000 round of censuses.<sup>12</sup> In some cases where neither source was available, data were obtained from reliable secondary sources that cite the original.<sup>13</sup> Some regions of the world provide significantly better data than others, and some simply do not exist in the public domain or even at all. While the data for Europe, the Americas and much of Oceania are of a fair standard, the data for parts of Asia and much of Africa are of more dubious quality. Annex 1 provides a summary of the raw data collected, showing whether the data are bilateral and the corresponding sources and years.

# **_4._ The Versions of the Data Matrix**

As noted above, four versions of the data are discussed, each progressively more complete but more speculative than the last. Each of the first three versions of the database contains two matrices covering both emigration and immigration for each of 226 countries or territories,<sup>14</sup> one recording the foreign-born, the other the foreign

> 12 In such cases, since the majority of censuses refer to the years 2000 and 2001 these years were prioritized; and where data were available for both 2000 and 2001, data for 2001 were preferred since it is more recent.

> 13 These include the Organisation for Economic Co-operation and Development, the Migration Policy Institute, the Economic Commission for Latin America and the Caribbean, the Department for International Development, the International Labour Organization, and the United Nations, together with various national statistics bureaux.

> 14 In addition to the 226x226 matrices, data aggregated into the 87 GTAP regions corresponding to the GTAP database version 6, may be obtained from the authors. This aggregation will likely smooth over some of the inaccuracies contained in later versions.

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