As others (and the article itself) note, first define a "city." Is it the arbitrary, and very political boundaries (of which each state, at least in the US has various and different rules how cities come to be, annexation, etc.)

For fun go google "world's largest cities". I guarantee you will get a different answer depending on the source, because each source defines a city differently. Of course there will be some overlap (Tokyo will be up there, along with a few other cities, but there is a rather large amount of variation). How can we not come to a consensus over something that seems so relatively easy to figure out? How difficult is it to count people in a city?

Again, what is a city? The political boundary? Is it the metro area? What type of metro area? The parts in which people commute to the center of "the" city for work? The surrounding counties that have economic ties to the "central" county? (This is roughly how the Census Bureau defines Metropolitan Statistical Areas, even though parts of the counties will have connections, but other parts of the counties will not). The urban area only? Pick whatever you want to call the city, just be consistent.

Let's say we go with a metro area, because it's more true of what a "city" is than the rather simple political boundaries. Which is most likely going to include if not other towns and cities, at least exurban areas that (economically if nothing else) act like an independent towns, and all manner of other interesting urban/non-urban forms of land use with people that all surround some sort of central place.

Let's say that you've defined your geographic area that constitutes a city.

Now let's get to the very difficult problem of defining the differences between urban, suburban and rural.

One of the more common answers to define "urban" is "population density," but then of course you must create a threshold for what constitutes high population and urban and what isn't. Are we talking about daytime population (workers) or nighttime population (residents)? Because there are still a major number of cities who's downtown will cease to exist after 5pm.

But defining what is center city/urban is actually not too terribly difficult, it's defining the edge cases that it becomes a difficult question. Where does the urban end and suburban begin? In most cities that are quickly growing, the old inner ring suburbs are now becoming quite urban in character. Old rural lands are now suburban. But where does one end and the other begin?

It's one of those things, like porn, you know it when you see it. You can probably point to one area and say "that's urban." "That's rural." But you (and me, and others) will all have slightly different definitions that while most will overlap, we will always disagree on the cases that could be both urban/suburban, suburban/rural or even (like parts of Detroit now) urban/rural.

If you can do all that, please give me a call, because congratulations, you're probably going to get nominated for a nobel prize in economics for precisely defining a city as well as coming up with a generalizable and replicable form of defining urban/suburban and rural.

(I could go on and on, but I won't but for extra fun, go read up on the Spersopolis. https://muse.jhu.edu/journals/sgo/summary/v035/35.2.hart.htm... )