By Katy Rossiter
The third installment of our geographic relationships series discusses geographic summary levels and how American FactFinder uses them to provide tabulated data.
To protect individual privacy, the Census Bureau provides summaries of data for geographic areas. For example, you cannot find the median household income for a particular house, but you can find the median household income for the census tract or county for where that house is located. Each summary level, identified by a unique three-digit number, represents a different geographic entity for which the Census Bureau summarizes data.
Summary levels specify the content and hierarchical relationships of the geographic entities so that the data can be tabulated. American FactFinder utilizes these summary levels so you can get to a specific piece of geography. When you are choosing your geography in American FactFinder, you are choosing which summary level you would like to use. For example, when you choose Madison County, IN, in American FactFinder, you are choosing to work with summary level 050 State-County.
|Summary Level||Geographic Component|
|150||State-County-Census Tract-Block Group|
The table above describes some of the basic summary levels that are also clearly shown in the geographic hierarchy. Census data are available for these summary levels and for all entities on the hierarchy. Census data are also available for some more complicated geographic relationships.
The table below shows some examples of relationships that are not clear on the hierarchy. These relationships are not as straightforward, and they require the Census Bureau to create special geographic units. These summary levels provide data for areas that might be difficult for the public to tabulate on their own so the Census Bureau does the work for you and makes these available in American FactFinder.
|Summary Level||Geographic Component||Description|
|070||State-County-County Subdivision – Place/Remainder||provides data for a place, but just the portion the place that falls in a specific county subdivision|
|158||State-Place-County-Census Tract||provides data for a census tract, or part of a census tract, that falls within a particular place|
|159||State-County-Place||provides data for a place, but just the portion that falls in a particular county|
There are a couple of keys to understanding summary levels. First, the last geography listed in the summary level is the one you will get data for and you need to know all of the proceeding geography in order to get to the correct geographic unit. Using summary level 060 State-County-County Subdivision, for instance, will get you county subdivision data but you will need to pick the correct state and county first. For example, if you want data for Oak Bluffs town, you need to enter the state (Massachusetts) and the county (Dukes) first.
Secondly, the most important component to the list of summary levels, like this one, is not the three-digit identifier but the hierarchical relationships between geographies. While the identifiers are useful and kept constant, the Census Bureau has introduced so many new summary levels over the years, the sequential order of the IDs is not very meaningful. More meaningful are the indentions on the list. These show how summary levels, and geographic units, fall within one another. American FactFinder makes the three-digit identifier for each summary level available in their search functions and you can pick your geography by searching for a particular summary level. American FactFinder also shows the hierarchical relationships in their search functions by also using indentions. An example is below.
Summary levels do not cover all possible geographic relationships. There are still some geographic units not covered by a summary level, and therefore, data users need to calculate the data themselves. For example, sometimes data users are interested in the population of a county that is unincorporated, in other words, the portion of a county that is not covered by a city, town or village. However, this summary level and dataset do not exist in American FactFinder, so data users would need to calculate the data themselves. They could use summary level 050 (State-County) to obtain the county population. Then, they could use summary level 159 (State-County-Place) to determine the population of each incorporated place that falls within the county and subtract the two.
Summary levels are one way to understand geographic relationships and understanding geographic relationships is important in order to get to the correct census data.
It is the 25 year anniversary of the TIGER (Topologically Integrated Geographic Encoding and Referencing) database. Stay tuned for more on this important milestone in geographic history.