On Nate Silver’s election live blog, he points out *“In every presidential election since 1960, the candidate who prevailed in Florida’s Hillsborough County, home to Tampa, has also prevailed in Florida”*

So, Hillsborough is a “Bellweather” for Florida – it’s been a reliable predictor for the Florida results for the past 13 elections. Now, exit polls are showing that Hillsborough is leaning towards Obama. Does that mean Obama will win Florida?

Nate points out “*There is no guarantee that the pattern will hold in 2012*“.

Why not? Have demographics changed? Or was it never a Bellweather to begin with?

The USA has about 3140 counties. Suppose each of them flips, purely by chance, towards this party or that, as do their states and the whole country. Purely by chance, then, half the counties will agree with their state result. Over the course of 11 elections, by chance one would expect 1 or 2 to have accurately predicted the state result every time. Likewise, there’ll be 1 or 2 that accurately predicted the national result.

Bellweathers can arise, then, purely by chance.

On the other hand, counties don’t flip randomly, nor do states. Some are staunchly Republican, others staunchly Democrat, others are swingers. The chance of a randomly chosen county predicting consistently the election result is probably much lower than 50%. Suppose only 1 in 5 counties can count as true flippers or swingers. Then, the pool of possible Bellweathers is much smaller. so the existence of a county that’s accurately foreshadowed the state result for not just 11 elections but 13 is quite remarkable.

Such a county must have a political “demographic” quite similar to its parent state. One can conclude that Hillsborough is like a mini-Florida all in itself – at least with respect to the people’s political views.

So bellweather counties exist. Nonetheless, there’s no county that can perfectly predict the outcome (unless we try to make the electoral college fairer and find a Penny) and we’d be unwise to make a prediction based solely on a supposed bellweather.

This cartoon from XKCD sums it up perfectly :