Measuring urbanism and density has a modifiable areal unit problem, that is, the statistic is difficult to define and easy to manipulate.

The largest problem is defining the geographic unit. As CityLab writes, if you analyze San Bernardino County's density, the average density is 101.5 people per square mile, but that metric doesn't take into consideration that the vast majority of people live in the Southwestern corner, since most of the county consists of the Mojave Desert

San Bernardino County, California, is physically larger than the four smallest U.S. states combined, and it includes a large swath of the Mojave Desert. Consequently, San Bernardino’s average density—100 people per square mile—is less than half that of Minnehaha County, South Dakota. But the vast majority of people in San Bernardino County are clustered in the county’s southwestern corner, in what is essentially an extension of the Los Angeles megacity. By contrast, Minnehaha County, though it hosts Sioux City, the largest urban center in the nation’s fifth least-dense state, also contains 1,000 farms averaging 353 acres each. Is it correct to say that San Bernardino County’s residents go about their daily lives in a world that feels, on average, half as dense as the lives of an average Minnehahan? Probably not: The Census Bureau includes most of San Bernardino’s population inside Los Angeles’s “combined statistical area,” which is home to 18 million people, while the largest statistical area around Sioux Falls contains only 230,000.

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