Based on an equation that can be used with available data and that provides a basis for facilitating decomposition analyses, this research estimates that about 2.54 million total (as opposed to enumerated) unauthorized Mexicans resided in the United States in 1996. Comparing this figure with an estimate of about 2.70 million released by the U.S. Immigration and Naturalization Service (INS) during the 1990s, we find that the two estimates involve different assumptions about circular, invisible, and ambiguous migrants. Such differences not only can have important policy implications; they can also be sizable and can operate in opposite directions, as illustrated by findings from a components-of-difference analysis. The results are also extrapolated to 2000, and implications for 2000 census counts are discussed.

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