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Isolated counties

Posted by Fitzaubrey on 20 Nov 2010 at 17:19 GMT

Some rural counties in the plains wind up with borders as if they are especially isolated. Grant County Nebraska, pop <800, looks especially strange. What is there about the methodology that causes this?

No competing interests declared.

RE: Isolated counties

cthiemann replied to Fitzaubrey on 21 Nov 2010 at 18:46 GMT

The isolated county in Nebraska (it's actually McPherson County, I think) has connections to southeast Nebraska and the part of Nebraska that is west of Colorado's east border, but is not connected to its geographical neighbors.

The community detection algorithm tries to maximize within-community traffic without any knowledge about the geography (i.e., it will try to place counties into the module to which it has the strongest connections). McPherson is essentially tied between two communities and due to the algorithm's stochasticity, it is sometimes assigned to the Colorado area even though its geographic neighbors clearly belong to the Nebraska/Iowa community. This causes the border around the county.

However, in this case we cannot conclude that McPherson is indeed isolated in reality: The number of dollar bill reports on is proportional to the county population. Data for these extremely small counties is sparse, and thus noisy.

No competing interests declared.