Thanks to the news this week, I spent some time over the last few days grappling with a question that many, many people have thought about before me: 'what's a suburb?' and what's our poll result for 'suburban voters?'

Our polls use a variety of different geographical variables in different contexts: census block group density, made up regions like 'philly suburbs', census-defined metro areas, or the NCHS urban-rural scheme. None obviously offer a consistent definition of 'suburb'

The best definition may vary on the purpose. But colloquially, I think that when people refer to the suburbs of 'x' city, they pretty explicitly do not mean any voters within central city 'x', regardless of density, and there are good political reasons to care about city limits.

So this certainly helps identify areas that aren't suburban, and in our last national poll, voters who live in the 'central city' of a metropolitan area (and for, say, the NY-Newark-Jersey City MSA, that's anyone within city limits of any three), Biden led 64 to 24 percent.

Though easy enough, this definition of 'urban' does leave something to be desired: there are plenty of 'central cities' that I'd say are part of the suburbs, like Alpharetta, GA.
There are also many tiny MSAs: Do we really want Dothan, AL as urban?
https://en.wikipedia.org/wiki/List_of_metropolitan_statistical_areas

One option here would be to limit ourselves only to central cities of a certain size (hoping to, say, keep SF and OAK but lose Alpharetta or Waukesha in ATL/MIL). This might also help with the small MSA problem. Still worth investigating.

On the other end of the spectrum is the challenge of delineating suburban and rural. It's easy enough to say the non-metropolitan counties are rural. But there are many metropolitan counties that are pretty rural, and there are certainly indisputably rural parts of metro counties

Here, the census does offers a possible solution: 'urbanized areas,' where the census generally considers non-urbanized areas to be 'rural.' I think this does a pretty good if imperfect and maybe outdated job of pulling out rural areas from MSA counties https://gis-portal.data.census.gov/arcgis/apps/MapSeries/index.html?appid=7a41374f6b03456e9d138cb014711e01#:~:text=The%20Census%20Bureau%20defines%20rural,rural%20based%20on%20this%20definition

Another option is some kind of population density floor, where a block group outside of the central city in an MSA county would be suburban if its population density is greater than x/sq mi.
That said, the urbanized area definition is close to many plausible density floors

I'm fairly satisfied with this end of the definition, though there are some high growth suburbs in the Sun Belt sprawling into 2010 'rural' areas and there are rich mansion zones that aren't urbanized by census standards, tho very few people there

If we leave our definition of suburbs here--census-defined urbanized areas outside of any central city in any MSAs--then the last NYT/Siena result for suburbs is Biden 50, Trump 34.

The biggest opportunity for improvement would be to see if there's a smart way to make the Alpharetta/Waukesha type 'central cities' count as suburbs, based on some kind of population floor. I'd guess it won't have a huge effect, but worth seeing if there's a non-arbitrary option

A secondary opportunity is to see if there's a way to grow our suburban definition to include post-2010 suburbs (possibly based on new ACS density data) or tiny non-urbanized rich enclaves (tough, as the pop density is quite low). I'd guess this isn't worth it or workable

Of course, every definition has shortcomings. And depending on the purpose, another may be more appropriate. But if I get one single, national standard definition of a 'suburb' for polling, I think something like this is most consistent with how it's used politically

The most coherent critique, IMO, is that this is a very industrial-northeast focused definition of a suburb. There are many Sun Belt metros where the city-limits are basically coterminous with the whole urbanized area, so we would have few no 'san antonio suburbs'

The way around it is to use a density/development based definition, which would make almost all the Sun Belt metros 'suburban.' I think this vision has merit and it also correlates well with vote choice.

OTOH, this remedy could put big chunks of Seattle, Queens and Detroit and wherever into the 'suburbs.' And for me that's kind of facially unacceptable, at least for this kind of generalized purpose. In this context, I'd settle for calling San Antonio urban.

And we can drill into these distinctions when appropriate. Central city block groups with a density >10000 sq/mi are Biden 74, Trump 14 in our poll; those under that are Biden 59, Trump 28.