Assuming the winter population has left the area entirely by March 31, the Census Bureau would estimate the county’s April 1 population at 100,000 and its July 1 population at about the same number. Therefore, in this very stylized example, both the ACS and a long-form sample conducted in the same year would provide a total population figure for the county of about 100,000 (since the ACS weighting procedure adjusts the ACS sample to conform to the July 1 county population estimates), but the composition of the population would differ between the two surveys. The long-form sample would provide an estimate of 20,000 poor people (20 percent poverty rate for the year-round population). The ACS would provide a 1-year period estimate of 15,000 poor people (15 percent average for the year-round and seasonal populations combined). Note that the percentage of people in poverty from the ACS estimate reflects the average composition of the population over the year; however, the number of poor people is lower than both the long-form-sample estimate and a 12-month average of the ACS that is not constrained to the July population control.

This example is exaggerated, but it does point up the differences between the long-form sample and the ACS for areas that experience significant seasonal fluctuations of population and for which the socioeconomic characteristics of the seasonal and year-round populations differ appreciably. In these instances, the long-form sample provides the numbers and characteristics of the population as of April 1. The ACS provides comparable population numbers by age, race/ethnicity, and sex based on July 1 postcensal estimates, even though the total population, as well as demographic groups (for example, young and older people), may change during the year. For socioeconomic characteristics, the ACS provides percentages that reflect the average experience of the area over the year; however, the percentages are applied to the July 1 population figures so that the numbers are neither the same as the long-form-sample estimate nor the same as an average estimate from the ACS that is not controlled to the census-based population estimates (see further discussion in Section 4-A.5).

For most areas, this problem will not be significant because seasonal increases (or decreases) in population are a small percentage of the year-round population, or the characteristics of seasonal and year-round residents do not differ appreciably. In areas for which users believe that seasonal differences may be significant, they may wish to make a case to the Census Bureau of the need for tabulations of their population at different times of the year (see Section 7-D.2).


Transportation planners are devoting considerable effort to understanding the ACS, determining how to work with the data, and identifying

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