I'm trying to answer a very simple question: by how much has the under 18 population in San Francisco changed since the pandemic.
The 2020 census redistricting data gives the population of San Francisco at 873,965, of which 760,738 were 18 years and over. That implies the U18 population was 113,227.
Table P12 in the Demographic and Housing Characteristics gives the same total and breaks it down by age and sex. So far, so good.
My understanding is that the Population Estimates Program is the place to go for intercensal estimates of the population count.
I downloaded the latest vintage age/sex breakdown by county. Here's the San Francisco data for select age buckets.
The total population figure in the base year is very close to the census figure, which is what I'd expect. Notice that the 0-17 population in the base year is estimated to 118,989 whereas the census gave a figure of 113,227. That's a 5% difference which is way beyond any margin of error.
Meanwhile, the ACS 1-year estimates are consistent with the population estimates:
Questions:
1) Why is the base year U18 population estimate so different from the census figure (118,989 vs 113,227)? Isn't the former supposed to be based on the latter?
2) Which number should I use? I want to use the 2020 census figure because then I've got a consistent series going back decades but comparing the census figure with the 2023 estimates implies that the U18 population fell by only 3,000 which is not credible for two reasons.
3) Even if we forget about the census completely, I struggle to believe the age breakdown of the population change implied by the ACS surveys and population estimates. Are we really to believe that a pandemic that killed mainly old people and led to 70,000 people leaving the city somehow caused the population 65 and over to increase by 8,500 and the population aged 75-79 to increase by nearly 25% in three years? If true, I would have expected this influx of retirees to be covered in the local media.
1. Estimates base is based on 2020 count, but a few things can contribute to differences. Programs like Count Question Resolution and Post Census Group Quarters Review create data that is incorporated in the Population Estimates Program (PEP) 2020 base. Unfortunately, the CQR and PCGQR results are not incorporated in the redistricting data or the DHC data. Separately, each time the PEP issues a new vintage of estimates, the PEP must inject Differential Privacy noise in the base, so each vintage will have a different base, a different launching point for the estimates. Assuming severe issues with CQR and PCGQR and DP noise, the differences generally should not be of the magnitudes you describe, but outliers should be expected.
2. When comparing population estimates to any other population figure, it's best to use the estimates base because that's what the estimates are based on. In some sense, the estimates base is based on the redistricting data/DHC data, but to the degree that the estimates base differs from redistricting and DHC, the PEP estimates base is the one to use. A whole separate issue is that the vintage 2019 PEP data was based on the 2010 Census, so the vintage 2019 PEP data is expected to be badly mismatched with the vintage 2023 data. If you only needed total population (no age detail), then you'd do well to wait until the November 7 intercensal estimates of total population (basically, 2010-2019 estimates will be revised). Intercensal age detail should be in the works, but it's not clear when or whether the sex, race, Hispanic detail will slow down the release of the age detail. (Some users rely on age detail much more than sex or race or Hispanic details.) It's not hard to imagine the number of children enrolled decreases much more than the number of under-18 residents decreases. The pandemic made school feel faraway and optional.
3. Regarding the 75-79 and 80-84, something seems off. The magnitudes you describe are larger (and over a shorter period of time) than would be expected as a result of aging baby boomers. The oldest (born in 1946) would be turn 77 in 2023, so some increase in the 75-79 population might be expected, but 19% in 3 years seems a bit much and baby boomers are not yet turning 80.