mySidewalk’s data library includes historical Census estimates alongside current data, enabling you to track changes over time. However, comparing data from different Census years is challenging due to changes in Census geographies.
Why Do Census Geographies Change?
Census geographies like blocks, block groups, and tracts are updated every Decennial Census to reflect population shifts. Here’s a breakdown:
Blocks: Smallest geography defined by physical features like roads and railways. These change to match the physical environment.
Block Groups: Aggregations of blocks, updated based on population changes.
Census Tracts: Larger geographies with some consistency over decades but may split, merge, or have new tracts created to accommodate population changes.
The Impact of Changing GEOIDs
The Census Bureau uses GEOIDs to identify geographic shapes. Changes to these IDs can occur in two ways:
Shape Adjustments: The GEOID may represent a shape that is now larger or smaller than before.
Reassignments: Rarely, a GEOID is reused for a completely different location.
How mySidewalk Harmonizes Geographies
To ensure consistent comparisons across Census years, mySidewalk uses geographic harmonization. This process “transposes” older Census data (e.g., 1990, 2000, 2010) into the current Census 2020 geographies.
Data Sources and Methods
mySidewalk provides data from:
Decennial Census (1990, 2000, 2010, 2020)
Current 5-year ACS estimates
We harmonize older data to 2020 geographies using Census block-level crosswalks provided by the National Historical Geographic Information System (NHGIS). This includes:
Block Group Reapportionment: Allocating data from older geographies to 2020 block groups.
Weighted Block-Point Apportionment (WBPA): Aggregating block group data to higher levels like states or cities.
For detailed methodology, refer to NHGIS documentation: NHGIS Geographic Crosswalks.
Citations
This article is based on data and methods outlined by:
Steven Manson, Jonathan Schroeder, David Van Riper, and Steven Ruggles: IPUMS National Historical Geographic Information System: Version 12.0 Geographic Crosswalks, University of Minnesota, 2017.