Inaccurate study harms the evidence-based immigration debate

Inaccurate study harms the evidence-based immigration debate
© Greg Nash

Last Friday, The Hill published an op-ed by Mohammed Fazel-Zarandi in which the author stated that prior estimates of undocumented immigrants living within the U.S. were far lower than the actual numbers. 

In fact, it is the study that the author referred to, “The number of undocumented immigrants in the United States: Estimates based on demographic modeling with data from 1990 to 2016,” that is way off, demonstrably so.

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The study’s model, which the Center for Migration Studies simulated, estimates that the U.S. undocumented population from Mexico increased by about 17.5 million during the 1990s.

That purported upsurge accounts for the entire difference between the study’s estimate of 22 million and the widely accepted 11 million estimate.

We tested this finding using Mexican population and vital statistics data from the National Institute of Statistics and Geography (INEGI) of Mexico. The INEGI data indicates that net emigration from Mexico from 1990 to 2000 was 5.5 million.

No reasonable assumptions of error in these statistics for Mexico would change the estimate of 5.5 million emigrants by more than a million or so. Clearly, 17.5 million could not have moved from Mexico to the United States in the 1990s because only 5.5 million were “missing” from Mexico in 2000. 

How could this study have overestimated undocumented immigration so badly? First, the study estimates arrivals from Mexico in the 1990s based solely on border apprehension data. The rates of apprehension (how many entered compared to the number that were caught) are purely speculative; they are not based on any empirical data.

Second, millions of apprehensions during the 1990s were of undocumented migrants returning to their home in the United States after a visit to Mexico. They were not additions to the U.S. population, although the study treats them as such.

Third, the study used inappropriate emigration rates of Mexican nationals from the United States. The rate the authors used to estimate emigration of short-term migrants — those that entered and left after less than one year — in the 1990s is the percentage that entered legally, overstayed temporary visas in 2016, and then departed. These two rates have nothing to do with each other.

We have shown that the study’s estimates of cumulative inflows are far too high and their estimates of cumulative outflows are far too low; we have also identified the primary reasons for those errors.

The information cited here was readily available to the authors of the study, and due diligence required that they conduct plausibility tests prior to publication of their paper, which failed in this case. In fact, it is clear that the peer review process itself failed. 

The authors have argued that the Census Bureau has far more dramatically undercounted the U.S. undocumented population than academics, researchers, demographers or government officials have ever recognized. This is inaccurate.

In fact, they do not want to be deported or to run afoul of the government in ways that might bring negative attention to them — ignoring a Census Bureau survey or the decennial census would do that.

The misleading estimates are already being used for ideological purposes, making an appropriate resolution to this challenging issue even more elusive. In the circumstances, it is important to rely on credible estimates of the undocumented population.

The well-being of millions of individuals — as well as U.S. families and communities — will be affected by the resolution of these discussions on immigration to the United States.

Robert Warren is a senior visiting fellow at the Center for Migration Studies. He served as a demographer for 34 years with the United States Census Bureau and the former Immigration and Naturalization Service (INS).