Migrant studies may involve several types of comparison, the most basic being between disease rates in migrants and in their country of origin, such as populations of similar genetic background but different environments or populations of different genetic backgrounds living in the same environment. 1 These "nature" experiments provide useful information in studying cancer risk factors or the cause of cancer in humans. Cancer diseases are responsible for about 12.0% of all deaths throughout the world, and 22.1% of all deaths occurred in developed countries in 1990. 2 Le et al. 3 compared cancer incidence rates between Vietnamese living in the United States (1988 -1992); residents of Hanoi, Viet Nam (1991-1993); and non-Hispanic whites.
However, these studies, in some cases, may suffer from serious selection bias; e.g., migrant populations are a nonrandom (selfselected) sample of the population of their country of origin. 1 Therefore, our aim was to comment on the report of Le et al. 3
CHARACTERISTICS OF THE STUDY
In April 1975, some 140,000 Vietnamese who were closely associated with the former South Vietnamese government were evacuated from the country and resettled in the United States. Thereafter, from 1975 to 1995, about 796,310 Vietnamese boat people entered different Asian countries and some resettled in the United States. 4 Because of the difficulty of these travel routes, very few children and elderly people could accompany the boat people, and this caused a non-normal age structure, e.g., Vietnamese Emigration 1979 -1989, compared to the general population of Hanoi City (Table I). In addition, Vietnamese living in the United States mostly come from southern Viet Nam. 3 These characteristics are strongly associated with the findings of the Le et al. 3 study because the natural age structure of the 2 populations is completely different.