It seems they are being smart about using counties:
"All analyses were carried out at the county level. Counties were combined as needed to create stable units of analysis over the period 1980 to 2014, reducing the number of areas analyzed from 3142 to 3110 (eTable 1 in the Supplement). For simplicity, these units are referred to as “counties” throughout."
They also use https://en.wikipedia.org/wiki/Small_area_estimation which seems like a way to deal with the counties with small sizes.
And from what I can tell they are using hierarchical models that should have some regularizing effect from larger populations.
They also use https://en.wikipedia.org/wiki/Small_area_estimation which seems like a way to deal with the counties with small sizes. And from what I can tell they are using hierarchical models that should have some regularizing effect from larger populations.