Guest Post: Census Geographies
To unlock the power of the US Census (data provided every 10 years) and the American Community Survey (data provided every year), an understanding of relevant underlying geographies is crucial. Below are explanations of the nine most relevant Census geographies.
Census Block: The smallest geography in the census, it represents a small segment of housing units in a neighborhood. Limited data is available, especially in lower density areas.
Census Block Group: A collection of census blocks are packaged together to create a census block group. Census block groups roughly represent neighborhoods.
Census Tract: This is the smallest unit in which population-level data from the Census is widely available. There are between 1,200 and 8,000 people in each census tract, but the optimal number per tract is 4,000.
Zip Code Tabulation Areas: This geography contains all addresses within a postal code. Zip codes can be found in parts of the county in which census blocks, census block groups, and census tracts may not be found. Zip codes follow existing geographic barriers such as county subdivisions and counties to a lesser extent than other smaller geographies.
County Subdivisions: This geography consists of cities, towns, townships, and villages within a specific county. These geographies are the smallest relevant self-governing geography included in the Census. The geographic boundaries of county subdivisions do not align well with other geographic boundaries such as Zip codes and census tracts.
Place: This geography is very similar to county subdivision, but it contains unincorporated areas in some states and is divorced from county of origin.
Counties: These are a largely administrative geography with a focus on governance. Counties consist of county subdivisions and places, and typically envelop urban high-density cities. This geography varies greatly in population across the United States, but is a strong choice for evaluative geography due to the relatively low standard error in Census and ACS metrics.
Metropolitan Statistical Areas (MSAs): This geography is based around core urban areas and encompasses all areas around this core that have a high degree of economic and social integration. MSAs are not limited by counties, places, or county subdivisions, but can cross relevant political boundaries in order to capture population interest.
Combined Statistical Areas (CSAs): This geography builds upon the concept of MSAs to include surrounding areas with less integration but high economic reliance on major metro areas. This could include hinterlands of major metros that are only semi-urban. CSAs create a boundary to identify regions that transcend metro boundaries.
In light of these definitions, we urge you to consider the geography of the question or problem that you have in mind. In many cases, we may need to consider establishing more localized data collection.
Written by Cameron Caputi, Director of Foundation and Community Analytics at Rochester Area Community Foundation. Cameron has a Master of Public Policy and Management degree from Carnegie Mellon University in Pittsburgh and is a skilled data scientist focused on data processes, dashboard creation, and advanced coding.