Last Tuesday, 5 of San Francisco city supervisor's seats are opened for election. San Francisco uses ranked choice voting. Each voter can mark multiple candidate in order of preference. In the initial round, if the top candidate did not get a majority of votes, it goes into instant run-off. The candidates with the fewest number of votes are eliminated. Its votes are transfered to the remaining candidates according to the next preference on each ballot. This process repeats until one candidate obtains a majority of votes among the remaining candidates.
This year 4 out of 5 of the district election has headed for instant run-off. The most crowded field are the 14 candidates competing for district 6 seat and the 21 candidates competing for the district 10 seat. The preliminary election result released (Nov 6) shows the epic battle of over 10 rounds of instant run-off before a candidate wins. To visualize how the process play out, I have charted the election data below.
The colored lines connects the candidates in each round. A circle marks the front-runner and a cross marks the candidates eliminated. Each time a candidate is eliminated, those votes are transfered to the remaining candidates. This "lift" up the line of the next round. In district 6 we can see that the relative position of the top 3 candidate are unchanged through out the process, with Jane Kim leading all the way. District 10 is more dramatic, with the front-runner status pass around the top 4 candidates. We see that Marlene Tran has propelled to the front-runner in round 15 after Teresa Duque was eliminated in round 14 (marked by the red cross). Interestingly no one but Tran has received much of Duque's vote. In a similar fashion, Dewitt Lacy's elimination lift up Maria Cohen. And Steve Moss' votes have passed mainly to Tony Kelly. Finally Lynette Sweet's vote goes mostly to Maria Cohen, making her the winner in under the preliminary result.
The election department is expected to release the final result in a few weeks. It will possibly change the result in a close race such as district 2. I will updated the chart once the data is available.
I have also made a first pass chart that I find not as informative.
To view the election chart it requires a web standard compliant browser with SVG support, e.g. Firefox, Chrome, Safari or Opera. IE 8 or below is not supported. Thank you.
2010.11.08 comments -