total number of emigrants is under-estimated, and in some cases one is (mis)led to conclude that 100% their immigrants to the OECD area immigrated to the US; as acknowledged by Carrington and Detragiache, this may approximate the reality for Latin America, but is clearly erroneous, for example, in the case of Africa. Third, the CD data set excludes South-South migration, which may be signi...cant in some cases (e.g., migration to the Gulf States from Arab and Islamic countries, to South Africa from neighboring countries, etc.). Finally, the de...nition of a migrant is simply a foreign-born individual residing in the receiving country; it is therefore impossible to distinguish between immigrants who were educated at the time of their arrival and those who acquired education after they settled in the receiving country; for example, Mexican-born individuals who arrived in the US at age 5 or 10 and graduated from US high-education institutions later on are counted as highly-skilled immigrants. Despite these various shortcomings, the CD estimates constitute a ...rst and very useful step toward building a fully-harmonized data set on migration rates by education levels.

In an attempt to extend Carrington and Detragiache’s work, Docquier and Marfouk (2004) collected data on the immigration structure by education levels and country of birth from most OECD countries in 1990 and 2000. They use the same methodology and de...nitions as Carrington and Detragiache (1998),<sup>3</sup> but extend their work in a number of ways. They use Census data or speci...c and detailed surveys for nearly all OECD countries: Census data reporting educational levels and countries of birth were used for 11 countries in 2000 and 8 countries in 1990. Survey data were used for 13 European countries. These data give the immigrants’ origin for all source countries, not only for the top 5 or 10 countries, thus preventing an under-representation of small countries, as was the case for the CD dataset. In addition, for 24 out of 30 receiving countries, the data collected also give the exact education structure of immigration, while CD had to extrapolate this from the US data, with the US representing only about 44.1% total immigration to the OECD. On the whole, the estimates are based on accurate information on the immigration stock per country of origin and education level (primary, secondary, and tertiary schooling) for 88.8% of adult immigrants in 1990 and 92.7% of adult immigrants in 2000. Hence, Docquier and Marfouk (2004) provide ”reliable” estimates (i.e., based on precise information for at least 80% of the total stock analyzed) for 175 countries in 2000 and 160 countries in 1990.<sup>4</sup> Aggregating over countries, the total number of adult immigrants living in the OECD area and aged 25 or more is estimated at 58.5 million in 2000 and 39.8 million in 1990. Emigration rates by education levels are obtained by comparing the number

> 3Their estimates are based primarily on national census and register data. Survey data were also used when census statistics were not available or not su¢ciently detailed. In a limited number of case where data by country of birth are not available, immigrants are de...ned as residents with foreign citizenship (e.g., Germany, Japan, Italy or Korea).

> 4At the same reliability rate, Carrington and Detragiache (1998) provided reliable estimates for about 25 countries.

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