# How to Interpret ACS 5-Year Estimates

I am not able to find a clear description of how to interpret 5-Year Estimates for ACS data.  In particular, I am not clear on what the values in question, in my case, population data (e.g., Population 5 to 9 years and 5 to 9 year olds enrolled in school) represent.  I am seeing conflicting information on whether the value (in this case, the population number) is an average over the 60 months, or what exactly it represents.

For example, in the document "Interpretation and Use of American Community Survey Multiyear Estimates" (https://www.census.gov/content/dam/Census/library/working-papers/2012/adrm/rrs2012-03.pdf) , it states, "An important property of ACS...estimates is that they do not represent a single point in time but an average [emphasis added] of the characteristics of a geography over a one-year, three-year, or five-year period, so they are referred to as period estimates. Data collected during the 60 months of five calendar years are combined together to produce estimates for the same levels of geography as did the Census 2000 long form."  So, this makes it sound like it is an average.

Meanwhile, in the document "A Compass for Understanding and Using American Community Survey Data: What Researchers Need to Know " (https://www.census.gov/content/dam/Census/library/publications/2009/acs/ACSResearch.pdf), it states, "While one may think of these estimates as representing average characteristics over a single calendar year or multiple calendar years, it must be remembered that the 1-year estimates are not calculated as an average of 12 monthly values and the multiyear estimates are not calculated as the average of either 36 or 60 monthly values. Nor are the multiyear estimates calculated as the average of 3 or 5 single-year estimates [emphasis added]. Rather, the ACS collects survey information continuously nearly every day of the year and then aggregates the results [emphasis added] over a specific time period—1 year, 3 years, or 5 years. The data collection is spread evenly across the entire period represented so as not to over-represent any particular month or year within the period." This is pretty clear that it is not an average, but is an aggregate (whatever that means).

The most recent version (2020) of the document "Understanding and Using American Community Survey Data: What All Data Users Need to Know" (https://www.census.gov/content/dam/Census/library/publications/2020/acs/acs_general_handbook_2020.pdf), only says that, "While an ACS 1-year estimate includes information collected over a 12-month period, an ACS 5-year estimate includes data collected over a 60-month period.", but does not give any more detail on what that means.

Any clarification on how to interpret and write up the 5-Year estimate data would be appreciated.

Parents
• My interpretation of the literature provided is they don't take the average of monthly percents for single year or multiyear estimates, nor do they take the average of yearly percents for multiyear estimates. Doing so would give a different (and incorrect) result than tallying (aggregating) all responses in the positive for a variable over the entire period and determining what percent that is of the total.

I provide an example (Building Permits Survey data, not ACS) in the table below that shows housing units permitted in Raleigh by year during the 5-year period 2018-2022. Averaging the respective yearly percents gives the incorrect answer (27.7) if one wants to know the percentage of all permitted units in the 5 year period that were allowed to be single-family units. The correct answer (25.3) comes from dividing the aggregate (sum) of single-family units in the period by the aggregate of total housing units in the period, and converting that to a percentage by multiplying by 100.

 Place Year Total housing units Single-family housing units Percent of units that are single-family units Raleigh 2018 4,211 1,304 31.0 Raleigh 2019 1,207 380 31.5 Raleigh 2020 3,479 1,162 33.4 Raleigh 2021 6,487 1,354 20.9 Raleigh 2022 8,635 1,875 21.7 Total = 24,019 6,075 138.4 Average of the 5 yearly percents = 138.4 / 5 = 27.7 Percent of 5-year aggregate = 6,075 / 24,019*100 = 25.3