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US Census and American Community Survey

Information on the US Census, American Community Survey: History, data, mapping, and more.

Aggregate Data

Aggregate Data

When you respond to a Census questionnaire, your personal information remains private for 72 years. So how is it that we can get so much information from the data products they produce?

It's because the data is released in aggregate format, meaning that information about individuals is released as counts or averages based on geography, time, or something else.

An example is data released about voting. I can't look up how my next door neighbor voted, but I can look up my voting ward and see aggregated data about how all of my neighbors voted and compare that to a different ward on the other side of town. This allows us to work with data about individuals without violating individual privacy.

One important thing to think about is population density and how that influences geographic aggregation. In areas with dense population, like the Milwaukee Metropolitan Area, aggregated areas can be quite small but still contain enough population to maintain privacy. In rural areas, where population is much more sparse, aggregated data is released that covers a much larger physical area. Continuing with the previous example about voting: If there are only two or three people in a voting ward, it might be very easy to determine how an individual voted.

While you might have heard about a Census Tract, there are many different types of geographies that the Census Bureau uses to release data in aggregate. Keep reading to learn more!

Legal and Statistical Areas

Legal vs. Statistical Areas

For statistical purposes, the nation is subdivided into two main types of geographic areas, legal and statistical.

Legal areas are defined specifically by law, and include state, local and tribal government units, as well as some specially defined administrative areas like congressional districts. Many, but not all, are represented by elected officials.

Example: State of Wisconsin

Statistical areas are defined directly by the Census Bureau and state, regional or local authorities, and include census tracts and urban areas. The primary purpose of statistical areas is to tabulate and present census data.

Example: Metropolitan Statistical area - "Milwaukee-Waukesha-West Allis MSA"


Differentiate between types of areas

Population Density, Milwaukee, Wisconsin

Hint: Expand the arrows at the upper left corner of the map to view the legend.

Polling Place Locations, Milwaukee, Wisconsin (2012)

Hint: Click on an area in the city to view information about the local polling place.

Milwaukee, Wisconsin Neighborhoods

Modifiable areal unit problem