About U.S. Census Data
The US Census of population and housing is an important resource for understanding places. The census provides information on diverse topics from population density, to rents and mortgages, to commuting modes and times -- thousands of variables that can be mapped. This document provides some background on what census data are and provides some tips for GSD students and faculty on how to get census data.
- Graphical Representation of Quantitative Data.
- Mapping Census Data Tutorial.
- Downloading Census Data
Census GeographyIn order to protect the confidentiality of individuals, the bureau releases only summary statistics for geographic areas: blocks, block-groups and tracts.
Population Density at three levels of geography
Census Blocks In urban areas, census blocks conform approximately to what we think of as city blocks. At this fine level of geography, the census only releases a subset of the data short-form questionaire.
Block Groups These areas are supposed to contain approximately 1200 people, but the actual count of people per block group varies widely. All of the short and long-form data is summarized at the block-group and tract level.
Tracts Tracts are larger than block groups.
The Decennial Census Survey
The US Bureau of the Census is responsible for collecting information used to redetermine allotments of governmental resources (including congressional representatives and education funding.) The constitution of the U.S. requires that in years ending in 0 each household of the United States receives a questionnaire. This 100 Percent Survey collects information on numbers of persons by age, gender, and ethnicity and for each household according to household composition and housing tenure. Between the years 1940 and 2000, the census bureau also sent out a long form to a smaller sample of households (1 in 6). This long form survey collected information on hundreds of variables including Income, Occupation, Housing Stock, Rent and Mortgage, Commuting behavior and many others. This long form survey was (unfortunately) discontinued for the 2010 census and replaced with the American Community Survey which is collected every year but with a much smaller sample size (less than two percent of the population).
- Census Bureau's Web Site for 2010 data
- Techincal Documentation for the tables of the 2010 Census of Population and Housing
Public Law 94: Residtricting Information
This is the first product of 100% count of the decennial census is used for redistricting. It is aggregated to the Block level and includes information about the occupancy staus of housing units and about race. Technical Documentation for the PL-94 Tables of the 2010 Census.
Demographic Profile Data
This summary of the 100% survey provides detailed breakdowns of population by sex, age, ethnicity, household type and tenure. This is the most succinct summary of the census at a block level. Tecnical Documentation for the Demographic Profiles
Summary Files 1 and 2
After releasing the PL-94 resitricting data, the census takes a year or two to process the 100% survey data to create useful cross tabulations that break down the population of each block by Age, Ethnicity, Houshold Composition, Housing Tenure. These data are released as Summary Files 1 and 2. SF1 and 2 provide very fine cross-tabulations such as houshold tenure by race and age. This can get pretty hairy but is usrful if you are interested in quantiative sociology. Tecnical Documentation for Summary File 1 and Summary File 2.
American Community Survey
In an effort to increase the frequency of "snapshots" of the american demographic landscape, the bureau began, in 2005 to collect data on a yearly basis. Rather than sending a survey to every household this program, called the American Community Survey, sends a very detailed survey to approximately about 1.7 percent of households each year. Naturally this sampling procedure produces wole-population estimates that are subject to error, and the confidence decreases for finer-grained geographies. Therefore data from a one-year sample is released only for Places having populations of 100,000 or greater. According to their methodology, by strategically comparing surveys from one year to the next, the census is able to increase the precision of the estimates, such that after 5 years of collection, estimates are released at the block-group level of agregation, along with estimates of precision for each measure.
- Data Dictionary for the 2005-2009 ACS 5 Year Estimates
- ACS Sampling Methodology
- ACS Sample Size and Data Quality
The 2000 Census Transportation Planning PackageOne of the greatest census products ever is the old Census Transportation Planning Package, for which the latest issue is from the 2000 census. The CTPP is based on the responses to questions about the morning commute from the long form survey. The unit of the survey is a houshold, but the journey to work questions ask about where people work, what mode of transportation is used and how long the commute took. Part 1 of the CTPP summarizes this information at the block group of the household. Part 2 of the CTPP tabulates this information by the block group of the commuting destination. Part 3 provides a matrix of the block-group to block group flows. For rmore information on using the ctpp, see Using the Census Transportaion Planning Package. Taka a look at some amazing maps that can be made with the CTPP! Part 1 of the CTPP provides a summary of the commuting patterns
Click Here to see smoe amazing products derived from the responses to the census questions about the journey to work.
Browsing Census Data
The bureau of Census has several tools for viewing and downloading data. Unfortunately they all seem to have been designed by someone who has no knack for interface design. Thankfully, the data are no encumbered by copyrights, and so the private sector has stepped up to the plate to produce decent web tools for exploring the data through on-line maps and downloading data.
Web based interfaces for the census can be used in several ways:
- For browsing the variables that are collected in the census
- Interactive mapping and finding demographic details of specific geographic areas
- Downloading data for detialed mapping and analysis
Browsing and Mapping data with American Factfinder 2
The first thing that American Factfinder tells you when you visit is "American FactFinder works with Mozilla Firefox 3.6 and Microsoft Internet Explorer 7. Other browsers may not perform as expected. " This is sort of a shame because these browser versions are several years out of date and it is very difficult to find and installthem even if you wanted to. This site is very dissapointing. On the bright side, at least the US Census does not try to prevent third parties from creating better access tools, and so the market provides. See the next section on Social Explorer.
The easiest way to explore the census on maps is Social Explorer. A commercial enterprise, makes many maps and feaures available for free. Many more are available to paid subscribers. Folks who have a Harvard ID and PIN can go to Social Explorer, Professional (courtesy of Harvard University Libraries.) TheMappong interface for Social Explorer provides an easy way to explore the contents of various census products, and to examine the broad patterns and query individual census areas. One thing to keep in mind when exploring data with social explorer is that some data items are available fro tracts and some others are available at the tract level. It is useful to browse around inb this interface before going to the download interface.
Obtaining Census Data for use in GIS
There are several pathways available for obtaining census data for use in GIS and other analytical purposes. This manual provides a page, Downloading Data from Social Explorer and American Factfinder that provides instructions for obtaining tables and geography files form the web for use in GIS. Admitedly, this is a troublesome workflow. If you are a GSD student and are content to use blockgroup level data from the 2000 census, you will find nation-wide tables of basic data in our shared file server goliath\public\geo\esridata_10\usa\census. If you are around during the day, you may want to check the Data services of the Harvard College Libraries.