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I am involved a little in both Vermont House redistricting and Burlington City Council redistricting.

I have noticed, at least two census blocks that the Census is clearly over reporting population. Both are visible using District Builder in this map of Burlington Vermont.

One census block is Block #16005, which is part of the University of Vermont "Green". Absolutely no buildings, residential or otherwise, and the Census reports 20 people.

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The other is Block #4460, which is the Ethan Allen Homestead with one caretaker (and perhaps a spouse or partner) living on the property. Yet the Census reports it as 12 persons. I know, for certain, that it's false.

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I presume they are transferring counts from neighboring blocks so that overall counts for cities, counties, and states, are still correct. I imagine they are trying to protect some private information by skewing these numbers. But is that the reason they are doing it? I really don't fully understand why they are doing this.

Has anyone else noticed something like this in their area? Does anyone know the skinny on it?

There are other really silly blocks that they have carved out, but that question can wait for later.

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    I do Census work for local government. The silly shapes is just how the Census divides up the areas, it is every street centerline, admin boundary, geographic boundaries (rivers) all combined together and then used as the block boundary, which leads to tiny areas or areas with no people. A household is very unlikely to straddle one of these boundaries, though I can think of an exception in my own city.
    – RomaH
    Commented Dec 28, 2021 at 21:33
  • Oh no, @RomaH , I haven't even gotten into the silly shapes yet. Up in the UVM Redstone Campus there are some extremely silly shapes. And there are all sorts of small tracts with zero population that are assigned a census block.
    – robert bristow-johnson
    Commented Dec 29, 2021 at 5:49
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    relevant: slowboring.com/p/census-privacy
    – eps
    Commented Dec 29, 2021 at 15:07
  • Thank you, @eps . It does appear to be quite relevant.
    – robert bristow-johnson
    Commented Dec 29, 2021 at 16:16
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    Well @Him, I was actually involved in redistricting. But the maps have been drawn for more than a year. I know that city and state governments can get confidential information from the census that private schlubs cannot.
    – robert bristow-johnson
    Commented Sep 29, 2023 at 14:24

1 Answer 1

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The Census Bureau practices disclosure avoidance to, as you suspect, protect the privacy of individual respondents. The Census Bureau wants to ensure that a consumer of the data it produces cannot de-anonymize the data they publish. They use a variety of techniques to accomplish that including both data swapping and noise injection either of which could presumably account for the issues you are seeing.

The linked page on disclosure avoidance contains a number of papers and other resources that go into much more detail about the different techniques that are used if you want to get into more detail.

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    @TechInquisitor - If you're injecting noise or data swapping, you need the aggregate results to remain correct. So it is entirely possible that they removed 20 people from a neighboring block and they just happened to appear in the vacant block (I don't know that they have special rules for handling completely empty blocks). Of course, it could be that there were some homeless people there or that someone entered their street address incorrectly and the mapping software put them in the vacant block.
    – Justin Cave
    Commented Dec 28, 2021 at 19:01
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    There are publicly available papers that describe the methodology and more if you keep digging, @TechInquisitor You should not be able to exactly explain why there are 20 residents there in any particular block because the noise injection should be random and more-or-less unexplained otherwise it isn't very good obfuscation. BUT, the aggregate data should still be correct, useful, with a known level of noise or error.
    – RomaH
    Commented Dec 28, 2021 at 19:14
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    @TechInquisitor The homeless encampments is absolutely a good reason to do it: otherwise census data could be used to track down illegally camping homeless people by looking at only the ones with > 0 population!
    – Joe
    Commented Dec 28, 2021 at 19:38
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    @robertbristow-johnson: somehow marking which empty blocks are safe to report as empty would require extensive knowledge of the blocks, would probably be politically fraught, and would defeat the purpose entirely. Any block not marked as "safe" could be assumed to have people living in it; further: any block removed from the "safe" list would certainly have people living in it. In both cases, the Census's greater interest in anonymizing data precludes the existence of such a list. Thus: protecting privacy requires the Census to report the UVM Green having about 20 residents.
    – minnmass
    Commented Dec 29, 2021 at 9:26
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    @robertbristow-johnson Another consideration is that even without disclosure control in play, data at this fine detail tends to be unreliable. If somebody fails to respond to the census, or a new apartment block gets built on what was previously empty land, 2020 counts can be wildly off a couple of years later. Local knowledge can help mitigate that, but it gets very labour-intensive if you're doing this in bulk. 2/2
    – G_B
    Commented Dec 30, 2021 at 6:39

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