Two weeks ago I noted that one principle of fair elections, transparency, can be in tension with another, the secret ballot. The tension plays out in current discussion as to how far election administrators should go in making public the data that underlie election results even when doing so might allow inferences about how individuals voted.

The controversy involves two different information formats often confused with each other that should be kept straight. Cast vote records are electronic summaries of individual ballots and the choices made on each, which may also carry additional information such as a timestamp of when the vote was received. These records can be of considerable interest to politics buffs. For example, they can reveal how persons who voted for a given candidate voted on other races—something you couldn’t necessarily deduce from conventional aggregated election results.

Ballot images go further: they are literal pictures of filled‐​out ballots, which means they also record stray marks and cross‐​outs, write‐​ins, coffee stains, and anything else that may have found its way onto the paper (you’re not supposed to write in the margins on your ballot, but some voters do, and even sign their names.)