tions. Errors in adjusted data include sampling error and biases in the adjustment procedure. Inaccuracy in block-level counts may also arise from imputation for nonresponse, proxy response, and other measurement issues. Users must be aware that block data are simply building blocks for larger areas for which relative accuracy can be better ensured.
The potential effect of census error on legislative redistricting is particularly hard to assess, given the intensely political nature of the process. The shrewdness of a mapmaker in piecing together blocks into districts arguably has more effect on any perceived bias in the district than do block-level census errors. However, it is certainly possible that high levels of error in the census could have major effects on districts within states. For instance, errors in the census might affect the urban-rural balance within a state, and any resulting district map could dilute the vote of urban residents at the expense of rural residents—or vice versa. Such outcomes would depend on the average size of the districts, the differential undercoverage rates of major population groups, the proportionate distribution among areas of these population groups, and the number of districts with high rates of census undercoverage.
Census data on age, race, ethnicity, and sex, which are asked of the entire population, have many uses, particularly as they form the basis of small-area population estimates that the Census Bureau develops for years following each census. Currently, the Bureau produces estimates of total population by single years of age, sex, race, and Hispanic origin on a monthly basis for the United States and annually for states and counties as of July 1 of each year. The Bureau also produces estimates of the total population every 2 years for incorporated places and minor civil divisions of counties (in states that have such divisions). Recently, the Bureau began producing biennial estimates of total population and children ages 5 through 17 for school districts. The estimates are produced by updating the census year figures with data from such sources as birth and death records (see Citro, 2000e).
Census-derived population estimates serve a variety of needs of federal, state, and local government agencies and academic and