What is an 'undervote'?
Voters are not required to vote on every race on the ballot. They can vote in some races and leave others blank. Typically, nearly every voter will vote in the race at the top of the ballot—last November, that was the President. Farther down the ballot, they tend to leave more races blank.
When a voter skips over a race, and casts no vote for any candidate who in on the ballot or any write-in, that is called an 'undervote.'
Finer detail: It is also an undervote if the voter votes for more candidates than is allowed in the race, and declines to fix the problem if the ballot is rejected. An undervote should also be registered if the voter marks the ballot in a way that neither the voting machine nor an election official can tell the voter's intent.
Why is the undervote rate important?
A high undervote rate doesn't prove that the votes were miscounted, but it is a red flag that indicates the need for verification.
Most of the more serious, known miscounts in Wisconsin elections and in other states were apparent in a high undervote rate.
Some, like the miscounted Stoughton referendum of 2014, showed up as dramatic undervote rates. In that race, a programming error caused the machines to read none of the valid votes, making it look like 99.7% of the voters had ignored the referendum. (A separate malfunction caused one machine to count votes that were not really there, which is what provided that extra 0.3%.)
Others miscounts can reveal themselves in undervote rates that are not as dramatic, but still suspicious. For example, electronically tabulated vote totals from one voting machine in the City of Marinette last November 8 showed that it had counted no vote from 128 of 416 ballots--an undervote rate of 30.8%. Sure enough, this was discovered to have been an electronic miscount when examined in the recount.
Elsewhere, machine malfunctions such as overheating caused the voting machines to miss some of the votes.
Deliberate manipulation might show up as a high undervote rate, but it's unlikely any skilled-but-dishonest programmer would make the voting machines simply miss votes. More likely, the dishonest programmer would 'flip' votes--that is, have the machine add some of one candidate's votes to the other's total--so the fraudulent totals would not look suspicious.
Finer detail: Not every high undervote rate will turn out to be an error. For example, one ward in the City of Madison tends regularly to produce a higher undervote rate than other wards. In this ward, a large number of voters are nursing home residents who vote as 'permanent absentee' voters. Ballots are routinely mailed to them, without them having to submit a new request for every election--but only as long as they return the ballots. If they have no preference or interest in any of the races on the ballot, they will return an entirely blank ballot, just to maintain their status as a permanent absentee voter.
How do you calculate an undervote rate?
To calculate the undervote rate for any race, add up the votes counted for all the candidates in that race.
Subtract that number from the number of ballots cast. That gives you the number of ballots from which no vote was counted.
Divide the number of blank ballots by the total ballots, and you have the undervote rate.
This link will give you the total number of votes cast and counted in last November's presidential election, by ward, in Wisconsin.
This link will give you the total number of ballots.
That sounds easy. Why do I need any more instruction than that?
It should be easy, but when calculating undervote rates in Wisconsin, we have a special challenge.
Believe it or not, in many counties, including Racine, the election officials don’t count all the votes—deliberately. We don't really know how many total votes there were.
The uncounted votes were write-in votes for protest candidates, like Bernie Sanders. Wisconsin law was amended in 2015 to remove the requirement that election officials count protest votes. They are required to count only the votes for candidates who are on the ballot and write-in candidates who have registered. Counties that want to continue to provide full election results have the option of counting and reporting the protest votes, which our election officials call--somewhat dismissively--'scattering' votes.
Setting aside the question of whether those votes should be counted, the fact is that in last November's election only some Wisconsin counties did count them. Based on the figures reported by the counties that reported all the votes after the recount, we can estimate that almost one in every hundred Wisconsin voters--0.955% to be precise--cast a scattering vote for president.
If you are working with the election results from any county that did not count or report scattering votes (Racine County did not), you need to increase the total number of votes by a number equal to 0.955% of the total ballots. This will be an as-good-as-you-can-get estimate of the actual number of votes for president.
What was the statewide undervote rate in last November's presidential election?
When adjustments are made to estimate the number of scattering votes for those counties that did not count them, we can calculate that 0.77% (about three-quarters of one percent) of Wisconsin voters, or about 1 in every 130 voters, cast a ballot without a vote for president.
Undervote rates significantly higher than that, in any individual jurisdiction, should cause election officials to look more closely to make sure that all the valid votes were counted.