I had a brief meeting yesterday with Jed Kolko, the Under Secretary for Economic Affairs at DOC, during which he alerted me to a major change for CT geography.
As of spring 2022, CT's eight counties have been replaced by nine COGs/Planning Regions. These will be used beginning with ACS and Population Estimates data released next year (ACS 2022 1- and 5-year data).
The new regions will be treated as "county equivalents" for statistical purposes, although the county-equivalent designation will also make them eligible to apply for federal grant programs open to counties. The new regions will get new FIPS codes.
Obviously this will impact any county (county-equivalent) longitudinal data comparisons for the state.
I have heard nothing about this until Mr. Kolko told me about it. It seems pretty important though.
Here's a short PDF explaining the change: https://www2.census.gov/geo/pdfs/reference/ct_county_equiv_change.pdf
Much longer FR notice, with a map, population comparisons, and new FIPS codes: https://www.federalregister.gov/documents/2022/06/06/2022-12063/change-to-county-equivalents-in-the-state-of-connecticut
-GR
Following up, the Bureau's listing of Substantial Changes to Counties & County Equivalents now includes crosswalks between the old and new Connecticut counties at the county subdivision level.
The good news for those looking to measure county-level changes: both the old counties & new county equivalents are entirely town-based, so it's possible to tabulate older data for the new units, or newer data for the old units, by summing county subdivision data with these crosswalks. The bad news, of course, is that for years to come this solution won't be obvious to many users, and it'll always be an extra step. This will also complicate change measures for all sub-county summary levels (census tracts, block groups, etc.), and the Bureau has not yet started providing relationship files for intercensal changes in those units. I hope they consider doing that soon! We eventually also plan to add intercensal small-area crosswalks to the IPUMS NHGIS collection of geographic crosswalks, which so far only cover decennial years 1990-2020. This Connecticut overhaul certainly provides added incentive for us to do that!