Should I compare data year-over-year?

The US Census Bureau recommends against comparing data across the Census’ American Community Survey 5 year datasets (2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 and 2021) with overlapping years.

The data estimates are from the Census Bureau’s 5-year American Community Survey that is produced by combining data across time periods. The ACS’s ongoing survey collects data all year round to produce 1-year estimates, and over 5 years to create the 5-year estimates. These estimates represent the characteristics over an entire data collection period.

If you need data from the 5-year dataset, you wouldn't normally see much change between 1 year of data. This is because the estimates are based on 4 of the same survey years and only have 1 unique survey year. Some clients prefer to wait 2 to 3 years before updating to the latest data. Other clients prefer to get annual updates so that they can say "we are using the most current US Census data available."

  • For the 2021 dataset, surveys were collected in 2021, 20202019, 2018, and 2017. Then, the later year survey responses were projected forward (e.g. 2017 income was projected into 2021 dollars). 
  • The 2020 dataset is made up of surveys that were collected in 2020, 2019, 2018, 2017, and 2016.

The weird thing about Census data for zips/ZCTAs this year is that the Census Bureau updated the zip/ZCTA boundaries for the first time in 10 years. And due to the boundary updates, you're going to see more changes than usual for zip data.

A quick example of these boundary updates is for a group of zips in Frisco, Texas.
Example Interactive Map:

2020 boundaries
2021 boundaries

Comparing overlapping data also does not measure what you may be seeking. The difference between overlapping estimates is essentially measuring the difference between the nonoverlapping portions.
For example, comparing the 2020 5-year ACS to the 2021 5-year ACS does not measure the change between 2020 and 2021, which is what you may be interested in knowing. Since the 5-year data represent a longer timeline, they are not likely to capture abrupt changes.

We tend to pull 2021 (most current), 2016, 2011 (5 year without overlapping collection years) and 2000 data for these types of projects.


Bureau, US Census. “Period Estimates in the American Community Survey.” The United States Census Bureau, Accessed 30 Mar. 2022.

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