# **_The Comparability of Migration Statistics_**

Compilation of comparable data across countries is hampered by the many disparities in collection practices. These include surveying on different dates, omitting different categories of people,<sup>10</sup> defining migration differently, a lack of standardization between the questions asked during the census, and alternative country coding used to record the responses.<sup>11</sup> The United Nations (1980) recommends that migrants should be enumerated according to the borders in existence at the time of census or survey. Over time, however, borders are redefined, and new countries are created; those interviewed may simply be unaware of boundary changes and misclassify their country of birth. Indeed a foreign-born migrant, resident for many years, may classify themselves as native even when they should in fact be otherwise classified, thus biasing results. In other instances, the non-response level of migrants may be very significant as census takers are often poorly trained and many mistakes will go undetected. During the 1969 Kenyan census, for example 78,756 individuals did not respond to the place of birth question, a high figure relative to the 158,692 that stated they were foreign-born (Bilsborrow et al 1997).

The problem of measuring illegal immigration is also considerable and the omission of illegal immigrants will distort official migration figures. The extent to which census data record them will vary across countries, not least with the likelihood that census returns could be used to detect and apprehend people without proper authorization. A pertinent example is Russia, a "migration magnet" in recent years and the country with the longest border, with approximately 450 official border posts. Feasible calculations estimating the number of illegal migrants residing in the Russian Federation range from 3.5 to 6 million persons (Heleniak 2002), though there is no real way of ever accurately knowing. In the United States, which arguably conducts the most comprehensive census, the best estimate of the number of unauthorized persons is calculated using a residual method, i.e. subtracting known authorized persons away from the total number of foreign-born (Passel 2005) . Numerous migrants are therefore likely to remain unrecorded, not least because it is not in their interest for the authorities to know that they are residing illegally. In some cases, it is also not in the political interests of the authorities to know or recognize the extent of the problem of illegal entrants.

The rate of naturalization also negatively impinges on the quality of migration statistics. This varies significantly among countries, and differences in a country's propensity to offer citizenship impacts potentially very seriously on migration statistics recorded by nationality. Naturalizations in South-East and Eastern Asia, for example, are rare; so much so, in fact, that in many cases foreign nationality data can be directly equated to the foreign-born. In these countries, first-generation migrants and additionally immigrant births contribute almost solely to the foreign population. Indeed, in the specific case of Japan, migrants of even older generations still need to apply for citizenship, a lengthy and arduous process. Owing to increased marriages between Japanese and foreigners, the number of naturalizations has increased steadily, though the figure is still comparatively very low. Conversely, in countries such as Belgium, where naturalizations are far more common, additional migrants and immigrant births probably contribute

> 10The United Nations notes, for example, that developed countries generally include refugees in their censuses whilst these are largely omitted from censuses in developing countries.

> 11 For example, the number of Puerto Ricans in the United States will not be recorded since they count as Americans.

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