Data Methodology

The Census Bureau’s Population and Housing Estimates Program (PEP) data by county includes details like counts by age, race, or ethnicity and goes back for decades. But how the Census Bureau reported and grouped those populations changed over time.

Race categories

Users will notice that the race categories change depending on the years selected in this interactive tool. This occurs because the Census Bureau has changed the race and ethnicity categories it makes available. To allow for comparisons over time, the race categories change depending on the earliest year selected in the comparison tool.

If the earliest year selected in the tool is from before 1990, the data only includes three race categories: 'white', 'Black', and 'other'. As a result, any comparison that includes data from before 1990 only includes these three race categories. Race categories other than 'Black' and 'white' are included in the 'other' race category for years after 1990 when comparing to pre-1990 data.

Any comparison where the earliest year is between 1990 and 1999 includes two additional categories: 'American Indian/Alaska Native' and 'Asian or Pacific Islander.' Separate reporting for 'Asian' and 'Hawaiian Native/Pacific Islander' are combined for years after 2000 when the comparison year is in the 1990s.

Data from 2000 onward considers 'Asian' and 'Hawaiian Native/Pacific Islander' as separate groups and also includes the 'multiracial' category. These categories do not exist for earlier years and do not appear in comparisons in this tool if a year prior to 2000 is selected. Prior to 2000, the Census Bureau did not separately identify people who were two or more races. All persons were grouped into singular race categories. In 2000, the Census added the 'Two or more races' category to the data. The Census Bureau states that the number of people in the separate race categories (i.e., 'white', 'Black', etc.) was impacted by this change as some people who would have previously been grouped within a single race category were grouped into the two or more category with the change. Pre-2000 and post-2000 data comparisons will result in lower values for the separate race categories in proportion to the 'two or more race' population.

Ethnicity categories

In addition to the changes in race categories over time, the Hispanic ethnicity also became available at the county level beginning in 1990. People of Hispanic ethnicity may be of any race. To consider Hispanic people as a distinct group, the tool above defaults to excluding Hispanic people from the race categories when the comparison years selected are both from 1990 and later. The resulting race/ethnicity comparison groups are: "Black, non-Hispanic", "white, non-Hispanic", "American Indian/Alaska Native, non-Hispanic", "Asian or Pacific Islander, non-Hispanic" and "Hispanic". There is also an option for users to hide the distinct Hispanic ethnicity, which then allocates Hispanic people to their designated race category.

Census reporting and update cycle

The Census Bureau releases annual provisional population estimates based on the previous decennial census and other data on births, deaths, and migration/immigration. Every decade, the Bureau reconciles these estimates and releases final data.

These provisional estimates are 'postcensal estimates', and the final estimates are 'intercensal estimates'. USAFacts used the final intercensal estimates for 1970 through 2009 and the provisional postcensal estimates for 2010 and after.

The most recent county-level data available by age, race, sex, and ethnicity are the Vintage 2020 Population Estimates (census.gov) for 2010 to 2019 and the Vintage 2022 Population Estimates (census.gov) for 2020 through 2022. We will update this experience, including the 2010-2019 estimates, when the Bureau releases county-level 2010-2020 intercensal estimates by age, sex, race, and ethnicity.

Use caution when interpreting population changes that use different estimate vintages. The 2010-2020 postcensal estimates are known to underestimate the population by about 1% nationally. This underestimate is, effectively, zero for 2010 and grows each year to reach 1% by 2020. The estimate years differ from the base 2010 decennial census; underestimates will be resolved in 2023 when the Census Bureau releases its 2010-2020 intercensal estimates.

Geography changes

In 2022, the Census Bureau accepted a new county-equivalent map for the state of Connecticut to better reflect the actual governance system in the state. This resulted in a new map that divides the state into 9 counties in place of the prior 8-county map. This presents a significant hurdle for providing context to Connecticut's state population changes over time. The Census Bureau, in addressing this concern, has indicated that they will release alternative population estimates for Connecticut for the past 5 years using the more recent 9-county designations. USAFACTS will be paying attention to those releases to determine if those results can be combined with these other data to provide a time series of population change for the new counties. While this is being determined, we have inserted the data from the Vintage 2021 Population Estimates (census.gov) for reporting for Connecticut at the county level, that align to the old, 8-county system to provide that context over time. State and National numbers use the 2022 Vintage estimates and we will continue to use the most recent estimates for the state and nation even when older data must be substituted for the county-level data. Until some additional data becomes available and is evaluated, we will limit Connecticut's county-level data to 2021.