Sooner or later, every urban planner or public health practitioner finds themselves needing to work with census data. Many people are uncertain as to how the census characterizes various facets of geography. There are also complaints that the census defined geography doesn’t fit an individual’s idea of what it should be in a particular area. Part of that comes from the need to have national standards; it is perhaps too much to expect that a national standard would exactly apply to every individual area. Perhaps that is a topic for a future blog post. Today, I am going to go through the various levels of census geography, starting from the most local. I am not going to cover every level reported, there are far too many different types of geography made available. The ones covered here represent those most often used in health and planning.
Blocks. Blocks represent the smallest area that the census makes data available for. In urban areas, these often correspond to actual city blocks, but in less dense areas, they may not. However, the census tries to have them bounded by actual physical features such as roads and railroad tracks. As valuable as the data might be on this very local level, most census data is suppressed at the block level because of confidentiality issues.
Block groups. As the name suggests, these are amalgamations of blocks (all higher level census geographies are collections of smaller areas). The block group aims to have about 1500 people, but this may vary. Note that all geographies can be of any size; in Alaska, a block group may cover thousands of square miles. They are sized for population, not land area. In general, most data are reported for block groups. They represent a finer grain of analysis than census tracts, but rarely do block groups correspond to anything that residents may find identifiable. In that sense they don’t represent neighborhoods, they are mostly analytical constructs.
Census tracts. These are the basic reporting units of the census. Again, they are sized for population, not land area. In my experience, the smallest are about 100 acres, the largest was over 10,000 square miles. The census aims for a population of about 4500 persons (or about 3 block groups). Tracts never cross county borders, their numbering system reflects this restriction. Many researchers talk about census tracts as if they were neighborhoods., but be very clear: they are not! They are drawn to try to correspond to some sort of on the ground reality, but the need to divide the country into tracts overrides any ability to reflect true neighborhood boundaries. In some communities, the tracts are much bigger than what locals consider to be a neighborhood, in others, much smaller. Though the “tracting” of the US began a hundred years ago (in part to assist health departments to understand local population numbers”, the tracking of the US did not become widespread until 1970 and was not fully implemented until 1990. Tract boundaries have also changed over time. In areas with growing populations, they have been split (I know of no circumstances where they have been combined – but that doesn’t mean that has not happened). Also, over time, the census has moved to rationalize tracts: make them more compact and eliminate incongruities. A tract with the same number in 2010 may not totally correspond to the same numbered tract in 1970.
Zip Code Tabulation Areas (ZCTA). Given the popularity of the US Postal Service’s zip code, the census has moved to define ZCTAs. Note that these are not exactly the same as zip codes. Zip codes are defined based on the needs of delivering mail, the ZCTA might be considered to be “rounded” zip codes adapted to make them fit the constraints of census geography. Many zip codes are for post offices or single office buildings. In general, these do not have a corresponding ZCTA. So don’t be annoyed when you can’t find data on every zip code in your community. Another issue is that ZCTAs (nor zip codes) can be relied upon to correspond to any way a neighborhood might be thought of. The reality is that there is no census definition of neighborhood or any census geography level that corresponds to what we might consider a neighborhood to be.
Counties. The US contains roughly 3400 counties and county equivalents. Even a geography that might seem as straightforward as the county contains a few quirks. Counties are called parishes in Louisiana and Boroughs in Alaska (I think). Some states, particularly Maryland and Virginia, have independent cities, not considered part of any county. Examples of these include Manassas Park in Virginia and Baltimore City in Maryland. These independent cities are considered and treated as if they were counties by the census. The District of Columbia, treated as if it were a state by the census, does not have any counties. Texas has the most. Note that even though counties in some states, such as Massachusetts have ceased to have any legal powers, they continue to be used by the census.
Metropolitan statistical areas. These are made up of one or more counties with a principle urban area (think center city) of at least 50,000. For the most part, but not always, these are what most people think of as metropolitan areas. They are defined based on commuting patterns and in consultation with state and local government. Sometimes, these definitions have been fairly static over time. In other metropolitan areas, they have expanded as the metro areas themselves have grown. In addition, the federal Office of Management and Budget (the official agency that defines MSAs – it’s not the Census Bureau that does it, sometimes defines new ones). Along with micropolitan areas, MSAs make up what are known as Core Based Statistical Areas. The names of these MSAs were changed prior to the 2010 census. In the past, they tended to be named after the single largest city in the MSA. Only a few MSAs, such as San Francisco – Oakland, had multiple cities in their names. Now, a large number of MSAs have multiple names. Most researchers and residents ignore the smaller city names and tend to refer to the MSA by its largest city.
Micropolitan statistical areas. These are similar to MSAs, but they are smaller in population, the center city has between 10,000 and 50,000 people.
New England City and Town Areas (NECTA). As noted above, counties have not real meaning in some parts of New England. Thus defining metropolitan areas based on county might not produce totally meaningful data. Thus in the six New England states, there are metropolitan (and micropolitan) areas based on amalgamations of cities and towns.
Metropolitan divisions. For some of the larger metropolitan areas, the census has defined subareas, called metropolitan divisions (in New England, there are also equivalent NECTA divisions). These consists of one or more counties. For example, The San Francisco MSA has a San Francisco metropolitan division and an Oakland metropolitan division (actually, again a metropolitan division can have multiple cities included in its name).
Combined Statistical Areas. Some metropolitan areas seem to be clustered together and highly economically integrated with their neighbors, yet each metropolitan area is still independent enough to make a complete consolidation imprecise. The way this has been accommodated is by the Combined Statistical Area. These can contain both metro- and micro-politan areas. Again, in there are equivalents in New England for the city and town based areas. An example is the San Jose – San Francisco – Oakland CSA. This CSA is a cluster of six MSAs.