Hi ACS friends,
I'm trying to explain to some brilliant but non-ACS-knowledgeable coworkers and others why the recommended approach is to use *non-overlapping* periods when making comparisons over time. I've looked at the Census doc (https://www.census.gov/programs-surveys/acs/guidance/comparing-acs-data.html) and all I can find is the guidance that says Do use non-overlapping datasets, Do not use overlapping datasets. I'm having a hard time finding the why. All I can come up with is this:
4-5ths of the sample is the same in adjacent datasets, e.g., 2014-2018 vs. 2015-2019. You can think of Census removing respondents from 2014, and adding respondents from 2019, but the respondents from 2015, 2016, 2017, and 2018 are the same. We should compare two completely different datasets, which means two non-overlapping periods.
I'm curious how others answer this question as well! -Diana
A colleague suggested looking at this paper (https://www.census.gov/library/working-papers/2012/adrm/rrs2012-03.html). There's a short discussion about non-overlapping periods in section 4.4 on page 9…
There are the following statements in the document https://www.census.gov/content/dam/Census/library/publications/2018/acs/acs_general_handbook_2018.pdf:
"TIP: As shown in Figure 3.1, consecutive 5-year estimatescontain 4 years of overlapping coverage (forexample, the 2010–2014 ACS 5-year estimates sharesample data from 2011 through 2014 with the 2011–2015 ACS 5-year estimates). Because of this overlap,users should use extreme caution in making comparisonswith consecutive years of multiyear estimates."
"TIP: In general, ACS 1-year data are more likely toshow year-to-year fluctuations, while consecutive5-year estimates are more likely to show a smoothtrend, because 4 of the 5 years in the series overlapfrom one year to the next.".
"When using ACS 1-year data, these comparisons aregenerally straightforward. Using multiyear estimates tolook at trends for small populations can be challengingbecause they rely on pooled data for 5 years. Forexample, comparisons of 5-year estimates from 2010to 2014 and 2011 to 2015 are unlikely to show much differencebecause four of the years overlap; both sets ofestimates include the same data collected from 2011through 2014. The Census Bureau suggests comparing 5-year estimates that do not overlap—for example, comparing2006–2010 ACS 5-year estimates with 2011–2015ACS 5-year estimates."