Chapel Hill Election Clustering

Damon Seils provided some great maps of the precinct results from this month’s local elections. I played around with the data, and found the results of a two-cluster analysis to be interesting. The ballots don’t include party affiliation, but candidates fell into two clusters, anyway, and the precincts fit several different profiles in support of those two candidate groups.

Two-way clustering of candidates and precincts
Two-way clustering of candidates and precincts

I’ll agree the diagram looks a bit complicated, but if you put in a little work, there’s a few gems to be found. Precincts are listed down the left side, and candidates across the bottom. The square at each precinct-candidate intersection is colored according to the candidate’s relative support at that precinct, red being strong, gray medium and blue weak. That part’s called a cell plot or heat map.

The tree-like parts are dendrograms, which show the results of the hierarchical cluster analyses. Similar items (precincts or candidates) are grouped together in the tree.

For the candidates, along the bottom, there’s a clear pair of clusters, which I’ll call left-leaning and right-leaning candidates. Coincidently, the left-leaning are on the left and the right-leaning are on the right.

The precincts are more interesting, though I have even less knowledge of their actual political orientations. I’ve colored the precincts into five groups. The first (red) and to a greater extent the second (yellow) cluster generally voted in favor of the right-leaning candidates. That is, the left six columns of the heat map are bluish and the right four columns are reddish. The opposite is true for the green and purple clusters; they’re more left-leaning, especially the purple precincts.

What puzzles me is the middle (blue) cluster. Those precincts don’t seem to like anyone. The numbers I used for clustering were percent of ballots cast, and apparently there were more voters in those precincts with incomplete ballots, voting in some but not all races. For instance, the two major mayoral candidates, Kleinschmidt and Czajkowski, only received votes on 28% and 21%, respectively, of the ballots at the Kings Mill precinct.

That leads to looking at votes per ballot for each race by precinct. Here’s a bar chart with the precincts ordered by town council votes per ballot.


Most precincts had near 100% participation in the mayoral race (exactly 100% for Booker Creek and Coker Hills), and most precincts averaged over three (of four available) votes in the town council race. So only the already-identified cluster of three (plus Dogwood Acres to a lesser degree) stand out regarding participation.

I imagine the One Stop (early voting for all precincts) totals reflect a lot of Carrboro voters. What makes the others different? Were people there to vote for a different race, like the school board? Or just voting for a favorite son/daughter candidate?

5 thoughts on “Chapel Hill Election Clustering

  1. I strongly recommend that you rebuild this interesting analysis to include the results from the Durham precincts. Email me offline if you have a problem finding access, but the results are on the Durham County BOE website.

    I’ve now been elected three times from my home in Durham County. For all three elections, the Durham County results, when added into the total, placed me one place higher than the Orange County totals. Chapel Hill has had voters in both counties for four decades, and the election totals must include the totals from both counties.

    The closeness of the margin in the Mayoral race results largely from the lopsided margin which Matt C had in Durham County, achieved largely through events at the Chapel Hill Country Club and in the Cedars retirement village. His three partners were evidently also part of these events.

    Look forward to meeting you sometime.

    Regards, Ed

  2. This is fascinating. I think the explanation for King’s Mill, One Stop and Patterson, and to a lesser extent Dogwood rests in the fact that many voters in those precincts do not reside within the city limits and could not have cast a ballot in municipal races. They could only vote in school board elections.

  3. Very nice, Xan. Your middle blue cluster can be explained by the fact that the Kings Mill and Patterson precincts include many voters who do not live within the Chapel Hill town limit and, therefore, were only voting in the school board election. This would also help to explain the One Stop vote, which likely included voters from these precincts and from other non-Chapel Hill elections.

  4. Patterson (northern part of town) and Kings Mill (southern part of town) both (and Dogwood Acres a little bit) have areas outside the city limits but within the school district. Therefore, it isn’t incomplete ballots but rather voters who only voted for school board.

  5. Thanks for the explanations of the seemingly low participation areas. And thanks Ed for pointing out my oversight in omitting the Durham County precincts. I’ll work on an update.

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