Many people know that I’m interested in the residual vote rate, which is the percentage of ballots that do not have a countable vote for a particular race. A residual vote is either an over-vote or an under-vote. Residual votes were at the core of the 2000 recount fiasco in Florida, where hanging or pregnant chad led to under-votes, and where weird ballot designs such as the “caterpillar ballot” caused over-votes.
What did the residual vote rate look like in the Jones/Moore race in Alabama? According to preliminary election results published by the Alabama Secretary of State’s office, of the 1,346,146 Alabamians who went to the polls last Tuesday, 1,779 cast no vote in the Senate contest. That’s 0.13% of turnout, which is a pretty small number. Still it’s not zero.
Why would a voter go to the polls in a hotly contested race and not vote in the contest? There are at least four possibilities: (1) The voter felt a civic duty to turn out, but couldn’t bring himself to vote for one of the candidates. (2) The voter made a mistake when marking the ballot and didn’t leave a legal mark. (3) The machine failed to read the legal mark. (4) The voter turned out to vote in another contest, and abstained in the hot contest.
It turns out that reason # 4 was a major driver of the residual vote in Alabama last Tuesday. We can see this in the following graph, which plots the residual vote rate for each county in the U.S. Senate special election. (Click on the graph for a bigger view.)
I have labeled the nine counties that are outliers. What sets them apart from the rest of the counties? With the exception of Choctaw, they all had other things on the ballot, generally taxation or millage questions. Considering how salient issues of local taxation can be, it’s not surprising that some Alabamians in the eight counties with referendum items would have shown up to vote about taxes, but not about the Senate.
How many voters might this be? No many. A back-of-the-envelope calculation suggests that only about 127 voters showed up in these 8 counties to vote for taxes but not for senator.
Similarly, it’s possible to use simple statistical techniques to estimate approximately what fraction of the residual vote rate was due to these additional ballot questions, or similarly, what would the residual vote rate have been if no counties had put tax questions on their ballots. The residual vote rate In the 59 counties without tax questions was 0.06%, compared to 0.51% in the eight other counties. A little algebra suggests that about half of the statewide residual votes in the Senate election were due to voters showing up for tax questions and skipping the Senate rate.
One final word about voting machines: The residual vote rate, if used properly, can be a valuable tool to diagnose inaccuracies in voting machines. One way this can be done is just by comparing the residual vote rate across different machines. In Alabama, if we do that with the Senate race, we see that counties using the DS 200 scanner had a significantly higher residual vote rate than counties that used the Model 100s (0.17% vs. 0.07%). However, this is entirely due to the fact that most (7/8) of the counties that had tax questions on the ballot used DS 200s. Controlling for whether a referendum was on the ballot in a county reduces the difference between scanners to virtually zero.
None of this, by the way, explains what happened in Choctaw County, which was the only outlier county with no tax questions on the ballot. It’s a very small county (4,240 voted), so that a small number of residual votes (18) can produce a residual vote rate that stands out by comparison. This might just be a “law of small numbers” issue, but it also might reflect something the county probate judge might look into.
Choctaw County, the only outlier with no tax questions on the ballot, is explained by the fact that the election results posted on the Secretary of State’s website don’t report any write-ins. However, the results reported by AL.com do show 17 write-in votes in Choctaw. So, it’s not an outlier after all. (Thanks to Justin Levitt for pointing out my error here.) Lowndes County also doesn’t have any write-ins recorded on the SOS’s website, although AL.com does. However, Lowndes also had three school tax questions on the ballot. Accounting for the 13 additional write-ins in Lowndes leaves it an outlier, just less of an outlier.