October 22, 2020

Ballot-level comparison audits: BMD

In my previous posts, I’ve been discussing ballot-level comparison audits, a form of risk-limiting audit. Ballots are imprinted with serial numbers (after they leave the voter’s hands); during the audit, a person must find a particular numbered ballot in a batch of a thousand (more or less).

With CCOS (central-count optical scan) this works fine: the CCOS prints the serial numbers consecutively, and the human auditor can easily find the right ballot in a minute or two. With PCOS (precinct-count optical scan), we are reluctant to print the serial numbers consecutively, because the order in which people insert their ballots at the polling place is visible to the public, and (in theory) someone could learn how you voted by correlating with the CVR file.

What about ballot-marking devices (BMDs)? How do the serial numbers work for use in ballot-level comparison audits?

First of all, let’s remember that RLAs of BMD-marked ballots are not very meaningful, because the RLA can only assure that what’s marked on the paper is correctly tabulated. Because most voters don’t inspect what’s marked on the paper, the RLA cannot assure that what the voter indicated to the BMD (on the touchscreen) has been correctly tabulated, if the BMD had been hacked to make it cheat.

But suppose we set that concern aside. And indeed, some jurisdictions are conducting “RLAs” on BMD-marked ballots. So let’s examine how such “RLAs” should work.

If the BMD prints a serial number onto the marked ballot before presenting the ballot for the voter to examine, then the voter can see the serial number, and can make a note of it. Then the voter can sell their vote, by telling the criminal vote-buyer the serial number. Or the voter can be coerced to do so. You may think this is a far-fetched scenario, but voter coercion and vote selling were common in the 19th-century and early 20th-century United States, and occurs now in some other countries.

Some “all-in-one” BMDs incorporate a scanning function, and don’t require a separate PCOS scanner. Suppose such a BMD prints a serial number onto the marked ballot after presenting the ballot for the voter to examine? That helps address the “voter-sees-the-number” problem. But it’s unpleasant to contemplate voting machines that can mark your ballot after the last time you see it. Any voting machine whose physical hardware can print votes onto the ballot after the last time the voter sees the paper,  is not a voter verified paper ballot system, and is not acceptable. But even so–suppose we permit this–we are in a similar situation to PCOS ballots. That is, the serial numbers should be in random order, not consecutive order, because otherwise observers in the polling place could calculate what serial number you’ll get.

And therefore, ballot-comparison audits of BMD-marked ballots run into just the same problem as audits of PCOS-scanned ballots, and maybe the same solutions would apply.

Because of this problem, some manufacturers of BMDs have done the same as manufacturers of PCOS: omit serial numbers entirely. For example, the ExpressVote and ExpressVote XL do not print serial numbers on the ballot*, and therefore their ballots (like PCOS ballots) cannot be easily audited by ballot-level comparison audits (except by a cumbersome “transitive audit”).

*Based on information about the ExpressVote and ExpressVote XL as configured in 2019 and deployed in more than one state, including New Jersey.