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American Community Survey

Cautions, Documentation and Guidelines on Using ACS Data

NOTE: Data are available for some areas, such as Baltimore City with a population of over 600,000, for all three data releases (annual, three-year and five-year). For Kent County, however, with a population of just over 20,000, data are available for three-year and five-year releases only. Data from the different releases should not be mixed, however, so if a comparison of median household income between Baltimore City and Kent County is needed for example, one would have to use the same data set - the three or five-year data for Baltimore City and Kent County to be consistent.

The Census Bureau collects ACS data from a sample of the housing unit addresses in the United States --rather than from the whole population All ACS data are survey estimates. To help you interpret the reliability of the estimate, the Census Bureau publishes a margin of error (MOE) for every ACS estimate.

American Community Survey 1-, 3-, and 5-year estimates are period estimates, which means they represent the characteristics of the population and housing over a specific data collection period. Data are combined to produce 12 months, 36 months or 60 months of data. These are called 1-year, 3-year and 5-year data.

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Distinguishing features of ACS 1-year, 3-year, and 5-year estimates
1-year estimates3-year estimates5-year estimates
12 months of collected data36 months of collected data60 months of collected data
Data for areas with populations of 65,000+Data for areas with populations of 20,000+Data for all areas
Smallest sample sizeLarger sample size than 1-yearLargest sample size
Less reliable than 3-year or 5-yearMore reliable than 1-year; less reliable than 5-yearMost reliable
Most current dataLess current than 1-year estimates; more current than 5-yearLeast current
Best used whenBest used whenBest used when
Currency is more important than precision Analyzing large populations more current than 5-year estimates Analyzing smaller populations Examining smaller geographies because 1-year estimates are not available Precision is more important than currency Analyzing very small populations Examining tracts and other smaller geographies because 1-year estimates are not available

For more guidelines on understanding and using ACS and sample Power Point presentations: Compass Products and Other Documentation