What is the "Vote for Highest Office" and "Total Ballots Counted"?
The Vote for Highest Office is the number of people who voted for the "highest office" in a given election. In presidential election years, the vote for highest office is (almost always) the presidential vote. In a non-presidential election year the vote for highest office is the largest vote total for a statewide office such as governor or US Senator. When no statewide office is on the ballot, the sum of the congressional races is used instead. In 2006, I changed this methodology slightly to use the sum of the congressional races if the votes for Congress exceeded a statewide office, as occurred with Indiana's uncompetitive US Senate race.
Some people do not cast a vote, even for president. Some failures to record votes are true errors, such as unrecorded votes originating from the infamous hanging chads of the 2000 Florida election. It is important to realize that some people intentionally abstain. for example, the 2004 presidential election 3,688 Nevadans voted for "None of These Candidates" (Nevada is the only state that allows this option). Under-Votes are such blank or indecipherable votes. Over-votes occur when a voter selects multiple candidates when only one is acceptable.
The best measure of participation is Total Ballots Counted, which includes all under-votes and over-votes. A problem is that not all states report total ballots counted. The national total ballots cast is estimated for missing states using a correspondence between the vote for highest office and total ballots cast for the states that provide both numbers. The good news is that an increasing number of states report total ballots cast, which allows estimation with greater precision of the national total ballots counted.
A statistic constructed by subtracting vote for highest office from the total ballots cast is known as residual vote.
The total ballots counted does not represent the true number of people who attempted to vote. Total Ballots Cast adds rejected ballots to the total ballots counted, such as rejected mail ballots or provisional ballots. These ballots are often rejected when a voter's eligibility is in question or the voter did not follow proper voting procedures. Reporting for total ballots cast is less frequent than total ballots counted, and I do not attempt to collect these statistics.