A single standard set of countries is specified for the entire timeframe of the database, for both origin and destination locations, so that migration numbers for pairs of countries can be compared over time. Since many new origin and destination countries emerged during the study period, the most current set of countries and regions was chosen.

A region is defined as any geographic entity that conducts its own census and that commonly features as an origin in the others‘ censuses. For example, Western Sahara is omitted because it does not conduct a census although it is a commonly designated origin region. In all, 226 countries, territories, and regions are included in this list in each of the five migration matrices (see appendix 1). One implication of these inclusion decisions is that migration from Croatia to Germany, for example, is reported in every matrix, even though Croatia did not exist in the early time periods. Researchers interested in migration from Yugoslavia to Germany in 1960 would simply total the individual migration levels from the successor states of Yugoslavia. Performing the analysis according to historical boundaries, though easier, would have masked many recent international movements. Moreover, drawing conclusions about destination countries that no longer exist would offer policymakers less useful information for drawing inferences.

Another complication is the 11 additional destinations with census data that do not map perfectly to the master list. Five of these were aggregated into other countries in the master list: Christmas Islands (to Australia), Cocos Islands (to Australia), Kosovo (to Serbia and Montenegro), South Yemen (to Yemen), and West Germany (to Germany). Six additional countries or territories no longer exist, but they map to two or more of the 226 locations on the master list. These are the Gilbert and Ellice Islands, the former Yugoslavia, Czechoslovakia, Ruanda-Urundi, the Trust Territory of the Pacific Islands, and the Soviet Union. Except for the Soviet Union, the census data for these countries or territories are disaggregated and distributed among the destination countries currently in existence on the basis of more recent migration figures.<sup>13</sup> All of these assignments are made according to the distribution of immigrants of the successor countries in later years.

The Soviet Union is a unique challenge. As mentioned, the enforcement of new borders and the creation of new nation states typically create new migrants overnight. According to the foreign-born definition, people who cross new borders that are created with the break-up of a country are considered migrants, even if they moved before the break-up while the country was still unified. This is particularly problematic in the case of the Soviet Union because 15 new sovereign nations were created overnight, there have historically been large numbers of internal migrants, and migrants have traditionally been recorded using a definition based on ethnicity. Failing to make any adjustment for the Soviet Union, therefore, would result in a large artificial jump in the number of migrants at the time of break-up (see appendix 3).

13 For example, the 1988 census data for the Trust Territory of the Pacific Islands were disaggregated and distributed among the Marshall Islands, the Federated States of Micronesia, the Commonwealth of the Northern Mariana Islands, and the Republic of Palau. However, in years when a country conducted its own census but was also included in the census of a more aggregated region, the country‘s own census is prioritized.

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