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20 March 2008

Primary Crosstabs

We're lucky to have two contested Presidential primaries. One of my favorite habits is to look at cross-tabs of candidate preferences by party and county. Here's an example of an Iowa cross-tab, showing the number of Iowa counties by Republican winner and Democratic winner:

We can visualize cross-tabs using mosaic plots as in "Visualizing Categorical Data." I did it for nine primary states in the image below. The green represents Obama counties, the orange Hillary counties and the purple Edwards counties. Across the columns are the Republican candidates: McCain, Romney, Huckabee. Across the rows, Obama, Hillary and Edwards. Check it out here. If you instead prefer an inverted version, with Republicans across the rows and Democrats across the columns (this makes it easier to compare the Democrats), check it out here.

The conclusions are the same over most states: Huckabee and Edwards are clearly the most complementary candidates. They shared counties whenever Edwards was in play (Iowa, Florida); after that, Huckabee shared Clinton counties. In Missouri every single county he won was a Clinton county! Huckabee and Clinton are somewhat complementary. Neither McCain nor Romney is particularly complementary with any Democrat (see California, where McCain and Romney split the Hillary-Obama counties), though both did better in Obama counties when Huckabee was in play.

One distracting feature of the plots above is that counties aren't uniformly populous. Obama won Missouri by winning only six counties. An alternative interpretation is to view this as an ecological inference problem, in which we are trying to determine the population totals in each of the cross-tab cells. This isn't perfectly accurate, since Edwards voters don't actually also vote for Huckabee. But it does provide a nice framework for scaling the mosaic plot by population size, and making it look generally less degenerate. I did that using Ryan Moore's eiPack and got this.

Posted by Kevin Bartz at March 20, 2008 5:53 PM