(see Box 3-1 in Chapter 3). Given their focus on issues of residence and intention to live in the United States, the ACS and CPS may also have problems covering people who have been in the United States for a short time (less than 1 year). They also appear to omit most of the seasonal workers, who usually live in Mexico and cross into the United States to work for a few months each year (a group that may account for a significant share, or even a majority, of unauthorized border crossers). Like all surveys tied to the decennial census, the ACS and CPS suffer from undercount. The undercount rate for unauthorized immigrants appears to be larger than for the rest of the population; estimates of the total unauthorized immigrant population based on the ACS (Hoefer et al., 2012) and the CPS (Passel and Cohn, 2011) make an adjustment for undercount in the range of 10 to 15 percent. Moreover, year-to-year comparisons have been complicated by the introduction of new population weighting methods in 2007 and 2008, redesigned questionnaires in 2008 (ACS only), and the switch to the 2010 Census as the base for weighting adjustments (in 2010 for the ACS and 2012 for the CPS).
The Mexican Census, ENOE, ENADID, and the MxFLS focus on Mexican households, a target population that is more relevant to this study than the U.S. households that are targeted by the ACS and CPS. One limitation of the Mexican Census, ENOE, and ENADID is that they miss entire households that have migrated1, thereby potentially underestimating flows from Mexico to the United States. ENOE will also miss whole households returning to Mexico because the migration information is only based on the second through fifth interviews; data from the 2010 Mexican Census suggest that about half of returning migrants return to households that did not exist prior to the return (Passel et al., 2012). The MxFLS, which tries to follow people when they move from Mexico to the United States, is less likely to miss the migration of entire households. However, only the MxFLS baseline sample (selected in 2002) reflects the national population in Mexico, and that sample is tied to the population at that time. Subsequent survey waves do not refresh the sample with new households and, hence, do not reflect the Mexican population at the time of data collection.
A more fundamental and general concern, however, has to do with the sample size of these traditional national household surveys. International migration is a relatively rare event, and it is important for a general survey to have a sample that is sufficiently large to obtain reliable information on
1 In the ENOE 2010, for example, an average of 3.5 percent of households were declared hogares mudados, or households who moved out between rounds. The reasons may be residential change in the same locality, internal migration, or international migration; we do not have precise information on the nature of the geographic mobility associated with the hogares mudados.