In my last post I described a particularly efficient kind of risk-limiting audit (RLA) of election results: ballot-level comparison audits, which rely on a unique serial number on every ballot. The serial number should not be preprinted on the ballot where the voter can learn it, otherwise the voter could sell their vote, or be coerced to vote a certain way, and the buyer or coercer could learn the vote from the file of cast-vote records (CVRs). The solution, when central-count optical scan (CCOS) is used, is that the central-count optical-scan voting machine can print the serial number onto the ballot, as it scans and counts the ballot. But many jurisdictions use precinct-count optical scan (PCOS): the voter marks a ballot, and feeds it directly into the PCOS voting machine, where it is scanned, counted, and preserved in a ballot box. This has three advantages over CCOS: PCOS machines can alert the voter about overvotes, undervotes, or blank ballots, which gives the voter a chance to correct their ballot. PCOS tabulations are ready immediately at the close of the polls, which gives faster election-night reporting. PCOS tabulations give an additional safeguard against low-tech paper-ballot tampering: if the hand-to-eye recount of this batch does not match the results claimed by the optical-scanner, then one of them is wrong. The paper ballots themselves are the presumed ballot of record; State statutes should say that in case of disagreement we trust the paper by default, not the (possibly hacked or buggy) computers; but even so, a disagreement is important evidence of possible tampering that could be worth a forensic investigation. We don’t get this safeguard with central-count scanning of precinct-marked ballots. But PCOS machines are not equipped with serial-number printers. Why is that? It would be straightforward to add one to a standard PCOS design, and it wouldn’t much affect the price of the product (so I’ve been told by the vice president of a major voting-machine company). The reason is not that the vendors can’t or won’t make the product; it’s that PCOS-ballot serial numbers are not so straightforward to use in RLAs.