The census is a social fact, the outcome of a process that involves the interaction of public laws and institutions and citizens' responses to an official inquiry. However, it is not a ‘hard’ fact. Reasons for inevitable defects in the census count are listed in the first section; the second section reports efforts by the US Census Bureau to identify sources of error in census coverage, and make estimates of the size of the errors. The use of census data for policy purposes, such as political representation and allocating funds, makes these defects controversial. Errors may be removed by making adjustments to the initial census count. However, because adjustment reallocates resources between groups, it has become the subject of political conflict. The paper describes the conflict between statistical practices, laws and public policy about census adjustment in the United States, and concludes by considering the extent to which causes in America are likely to be found in other countries.