Cite as: 525 U. S. 316 (1999)
Opinion of the Court
tion gathered from the nonresponding housing units surveyed by the Bureau would then be used to estimate the size and characteristics of the nonresponding housing units that the Bureau did not visit. Thus, continuing with the first example, the Bureau would use information about the 100 non-responding units it visits to estimate the characteristics of the remaining 100 nonresponding units on which the Bureau has no information. See ibid.
The second challenged sampling procedure—which would be implemented after the first is completed—is known as Integrated Coverage Measurement (ICM). ICM employs the statistical technique called Dual System Estimation (DSE) to adjust the census results to account for undercount in the initial enumeration. The plan requires the Bureau to begin by classifying each of the country's 7 million blocks into "strata," which are defined by the characteristics of each block, including state, racial, and ethnic composition, and the proportion of homeowners to renters, as revealed in the 1990 census. Id., at 30. The Bureau then plans to select blocks at random from each stratum, for a total of 25,000 blocks, or an estimated 750,000 housing units. Ibid. Enumerators would then conduct interviews at each of those 750,000 units, and if discrepancies were detected between the pre-ICM response and ICM response, a followup interview would be conducted to determine the "true" situation in the home. Ibid. The information gathered during this stage would be used to assign each person to a poststratum—a group of people who have similar chances of being counted in the initial data collection—which would be defined by state geographic subdivision (e. g., rural or urban), owner or renter, age, sex, race, and Hispanic origin. Id., at 31.
In the final stage of the census, the Bureau plans to use DSE to obtain the final count and characteristics of the population. The census plan calls for the Bureau to compare the dual systems of information—that is, the data gathered on the sample blocks during the ICM and the data gathered on
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