What’s a Residual Vote, and What’s It Doing in the Elections Performance Index?

Charles Stewart
MIT Election Lab
Published in
7 min readAug 29, 2018


What does it mean when a voter’s ballot doesn’t contain a vote for president? The answer to this question reveals what we know, or think we know, when we look at the residual vote rate, one of the seventeen indicators in the Elections Performance Index.

What Is the Residual Vote Rate?

But first, let’s review what a residual vote is and why it might be important. The measure was invented by the Caltech/MIT Voting Technology Project in the weeks following the 2000 presidential election, as a way to assess how many “lost votes” were caused by poorly performing voting machines.

A residual vote is either an over- or under-vote for a particular race, usually for president. The residual vote rate in a state is simply the total number of over- and under-votes divided by the number of people who turned out to vote. (Election-geek detail: Because most states do not separately account for over- and under-votes, the number of residual votes is usually calculated by taking the number of people who turned out and subtracting the number of votes cast for candidates.)

Generally speaking, the residual vote rises and falls over time for one of three reasons:

  • voters might abstain from casting a vote for that office at all;
  • voters might be confused and either fail to mark their ballot or mark too many choices; or
  • a problem with the voting technology might cause the voter’s choice to go unrecorded.

When the EPI was first developed, research suggested that the abstention rate for president was roughly constant over time and across states. Therefore, changes in the residual vote rate were most likely caused by fluctuations in voter confusion and technology failure. Because these both lead to lost votes, the inclusion of the residual vote rate in the EPI is intended to quantify the degree to which the machinery of ballot-marking and vote-counting serves voters, for better or worse.

How Does the Residual Vote Rate Vary?

Variability in Residual Vote Rates in 2016

In 2016, the residual vote rate ranged from 0.0039% in Nevada to 3.1% in Maine. (Nevada’s rate was the lowest observed since the EPI project began; more on that in a moment.) The accompanying map shows the states with the lowest rates (colored darkest green) and the highest rates (darkest orange), with shades in between. (The grey states on the map do not reliably and uniformly report turnout, and so are excluded from the calculations.)

Nationwide Residual Vote Rates from 1980 to 2016

The residual vote rate not only varies across states; it has varied across time, as well. Take a look at the accompanying graph, where I have plotted the nationwide residual vote rate since 1980. Two things bear noticing. First, the biggest drop occurred between 2000 and 2004, when it fell by nearly a full point. Second, there was a noticeable up-tick in 2016.

The stories behind these two big changes help illustrate what drives the residual vote rate, and what it can tell us about elections.

Why Does the Residual Vote Rate Change?

The residual vote rate is driven by the number of over- and under-votes in a presidential election. What would affect these numbers? Let’s start with under-votes.

There are many reasons why a vote doesn’t register on a ballot. Generally speaking, the most common reason is that voters abstain. They don’t like the candidates running for president, or they are indifferent between them. The less excitement there is about presidential candidates, the more under-votes there will be, causing the residual vote rate to rise.

The 2016 uptick in the residual vote rate is probably due to an increase in abstentions in the last presidential election. In a conference paper that I co-wrote with collaborators last January, we showed that more moderate voters — from both parties, but especially Republicans — were more likely to leave their choice for president blank. (The paper is being revised, but there is a link to it here. Note that an update will be posted shortly.) The result was a small increase in the residual vote rate of around half a percentage point compared to the recent past. This might seem trivial on its face, but when the residual vote rate has recently averaged around 1%, it’s a big deal.

Voter confusion and malfunctioning machines can also lead to under-votes. The punch card problems that were the centerpiece of the 2000 Florida recount are the best-known examples, but they aren’t the only ones. In old lever machines, for instance, the gears of the “odometer” counting wheels used to get stripped, causing some votes not to be registered, unbeknownst to the voter..

Over-votes can also be caused by poor ballot design or subtler machine problems. Since HAVA became law, most optical scanners have been required to return any ballot that contains an over-vote to the voter, but not all scanners are properly programed. Smudges can be inadvertently made on ballots, then inadvertently interpreted as over-votes when scanned.

A reduction in machine problems probably explains the one-percentage-point decline in the residual vote rate between 2000 and 2004.

In a paper Steve Ansolabehere and I published in the Journal of Politics in 2005, we showed, for instance, that just shifting from punch card to scanned paper ballots would reduce the residual vote rate in a county by 1.3 percentage points.

The years between 2000 and 2004 saw a huge shift in voting machine types, and that benefitted voters by capturing more of their votes. However, not all jurisdictions in America shifted from punch cards to paper ballots in these years. Thus, some of the reduction during that time, which persisted into the future, probably occurred because election officials just became more careful in counting ballots.

A Word about Nevada

One of the anomalies in the 2016 EPI is that although the residual vote rate went up significantly overall, it declined to a record-low level in Nevada. What is that about?

The short answer is: “none of these candidates.” In the 1970s, in the wake of Watergate, Nevada added this choice to its ballot. Unlike the rest of the nation, if a Nevadan doesn’t like the options on the ballot, she can still express that dislike in a positive way by voting “none,” rather than abstaining. The use of the “none” option skyrocketed in 2016, to its highest level since its first use on the presidential ballot in 1976.

Nevada Residual and “None” Vote Rates from 1976 to 2016

The accompanying figure shows the full story. In 1976, 2.47% of Nevada voters chose the “none” option. This is on top of the 2.20% of voters who chose no option at all, including “none.” If we add these two together, that sums to 4.68%, which is what the residual vote rate would have been in 1976, assuming the “nones” would have just abstained instead.

Over the next three decades, both the “none” choice and the residual vote rate in Nevada declined in parallel, until 2008, when a slight wedge was opened between the two. This gap opened up even further in 2016, when 2.56% of voters chose the “none option,” while the residual vote rate declined to a record-low 0.0039%.

The Role of the Residual Vote Rate in the EPI

The residual vote rate is one of the few indicators in the EPI that touches on the issue of the accuracy of the vote count. On the other hand, as 2016 reveals, the residual vote rate can change because of the nature of the presidential campaign in the run-up to the election, and in such a way that most would argue has little to do with election administration or policy.

The overall scores of a few states in 2016 would change noticeably if the residual vote rate were excluded. Montana would change the most if the residual vote rate were removed from the index, increasing by four points; Colorado, Maine, North Dakota, and Oregon would increase by three. Although these seem like small point changes, those small changes in points can yield big changes in rank because so many states are bunched up so close to each other.

(Of course, I have to add that we discourage a focus on rankings, but that is like King Canute commanding the tide.)

While a good argument can be made that the abstention-driven rise in the residual vote rate in 2016 had little to do with election administration and policy, the example of Nevada requires me to respond, “not so fast.” The residual vote rate proved itself as a valuable diagnostic tool in the 2000 presidential election, and the availability of the “none of these candidates” option in Nevada shows that election laws can be changed to allow an alternative to abstention on the ballot. I know that the idea behind the “none” choice was not to make it clear that a missing vote was likely due to some type of mistake, but we now know that this is a role that this choice can play.

Thus, perhaps the thing to learn here is that Nevada is onto something. Giving voters who would like a chance to express their displeasure with the nominees for president also has the salutary effect of making voting machine problems more obvious. That’s a policy choice that’s at least worth considering.



Charles Stewart
MIT Election Lab

Kenan Sahin Distinguished Professor @ MIT; Co-director of the Caltech/MIT Voting Technology Project; Founding director of the MIT Election Data & Science Lab