Census geography: Bridging data for census tracts across time

People: John Logan (Brown University), Zengwang Xu (University of Wisconsin, Milwaukee), Brian Stults (Florida State University), and Charles Zhang (University of Wisconsin, Whitewater). The tract data from ACS 2015-2019 were prepared by Rachel McKane (Brown University), and the LTDB standard data set for 1970-2010 was prepared by Miao Chunyu (University of Wisconsin, Stevens Point) from files downloaded from the NHGIS.

References:
Logan, John R., Zengwang Xu, and Brian J. Stults. 2014. "Interpolating US Decennial Census Tract Data from as Early as 1970 to 2010: A Longitudinal Tract Database" The Professional Geographer 66(3): 412–420. (Click here to download).
Logan, John R., Brian J. Stults, and Zengwang Xu. 2016. "Validating Population Estimates for Harmonized Census Tract Data, 2000–2010" Annals of the American Association of Geographers. http://www.tandfonline.com/doi/full/10.1080/24694452.2016.1187060.

The Longitudinal Tract Data Base (LTDB) provides public-use tools to create estimates within 2010 tract boundaries for any tract-level data (from the census or other sources) that are available for prior years as early as 1970 and now also for 2015-2019 and 2020. We also provide a Backwards LTDB in which data provided in 2010 tract boundaries can be estimated within 2000 boundaries. Researchers increasingly work with information aggregated to the tract level from non-census sources, such as criminal justice, public health, and voting records. To meet their needs requires a tool to convert such data to the 2010 boundaries. The LTDB offers an open-source crosswalk to link data from 1970-2000 to 2010, and it also provides user-friendly programming code to bridge data across years.

The LTDB is described in detail in Logan et al 2014 (see above). A subsequent article (Logan et al 2016) compares the LTDB estimates with those provided by Geolytics (formerly the NCDB) and another source provided by NHGIS. Based on our analysis we do not recommend the Geolytics data set. The NHGIS and LTDB estimates are very similar. Users should read the Annals article to make choices about what data to use and what precautions are needed when using any estimates.

2020 Update

The original LTDB is based on 2010 tract boundaries.  We have decided to maintain these boundaries for post-2010 data for two reasons: 1) The 2020 Census using 2020 boundaries are not yet available, except for the PL94 redistricting file.  When new data become available, we will harmonize it to 2010 boundaries.  2) The most recent ACS data are for 2015-2019, and these data are in 2010 boundaries.  Because the 2020 annual sample of the ACS was only partial, there is limited utility to the upcoming 2016-2020 ACS, which will use 2020 boundaries.  It will not be until 2021-2025 that a version of the ACS will be available that is not affected by the 2020 issues.

The downloads available now include ACS 2015-2019, and will soon also include 2020 race/Hispanic origin counts harmonized to 2010 tract boundaries.

LTDB-DP

Logan et al (2021) uses confidential microdata in the 2000 Census to test the accuracy of LTDB estimates for a selected set of full count and sample count data. It finds that assumptions made by all interpolation techniques, including the LTDB, involve large errors in a substantial share of tracts when estimating characteristics other than simple population counts. It also tests an alternative approach in which estimates are based on the original data, to which random noise has been added to create “differential privacy” (DP) estimates that can be disclosed. The DP estimates are far better. . The Census Bureau has now approved disclosure of the DP estimates, and we provide them in a new pilot system that we call the LTDB-DP. A request is pending to disclose DP estimates for all the LTDB variables for 2000.  A working paper describing this approach is:

John R. Logan, Charles Zhang, Brian Stults, and Todd Gardner. 2021. "Improving Estimates of Neighborhood Change with Constant Tract Boundaries" Applied Geography (132). doi.org/10.1016/j.apgeog.2021.102476.

Note that much of the U.S. was not divided into census tracts in 1970, and even in 1980 many less populated areas were not tracted. Consequently many 2010 tracts will have missing values for 1970 and 1980.

Evolution of tracted areas in the U.S., 1970-1990. Blue areas were tracted in 1970; areas in red were added in 1980. In 1990 the entire nation was divided into census tracts

For more information about the LTDB, select one of the following links:

• Use the LTDB and LTDB-DP Cl ick here for information about components of the LTDB, LTDB-DP, and file downloads
• Map the LTDB data using the web-based map system for 1970-2010 (ACS2015-10 and Census 2020 coming soon)