Cite as: 536 U. S. 452 (2002)
Opinion of O'Connor, J.
Bureau lacked household size information for approximately 0.2% of addresses that were classified as occupied. See id., at 191.
At this point, the Bureau employed the statistical technique known as "hot-deck imputation." For each unsuccessfully enumerated address, the Bureau imputed population data by copying corresponding data from a " 'donor' " address. Ante, at 458. The donor address was the " 'geo-graphically closest neighbor of the same type (i. e., apartment or single-family dwelling) that did not return a census questionnaire' by mail." Ibid. (quoting Brief for Appellants 7-8, 11). What this means is that donor addresses were selected only from addresses that had been personally surveyed by the Bureau's enumerators, primarily through the nonresponse followup procedure described above. See App. 156. After imputation was completed, every address on the master address list was associated with a household size number that had been determined either by imputation or by enumeration (although that number was zero for addresses ultimately classified as vacant or nonexistent).
The Bureau used the imputation-adjusted data to calculate state population totals. Ante, at 458. Because these totals were used to determine the apportionment of congressional Representatives, ibid., we must determine whether the Bureau's use of imputation constituted a form of sampling. If it did, it was prohibited by § 195 of the Census Act, 13 U. S. C. §1 et seq. See Department of Commerce v. United States House of Representatives, 525 U. S. 316, 338 (1999).
II
As initially enacted, § 195 provided that "[e]xcept for the determination of population for apportionment purposes, the Secretary [of Commerce] may, where he deems it appropriate, authorize the use of the statistical method known as 'sampling' in carrying out the provisions of this title." 13 U. S. C. § 195 (1970 ed.). As relevant here, Congress re-
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