Differing methods of measuring voter turnout can contribute to reported differences between nations. There are difficulties in measuring both the numerator, the number of voters who cast votes, and the denominator, the number of voters eligible to vote.
For the numerator, it is often assumed that the number of voters who went to the polls should equal the number of ballots cast, which in turn should equal the number of votes counted, but this is not the case. Not all voters who arrive at the polls necessarily cast ballots. Some may be turned away because they are ineligible, some may be turned away improperly, and some who sign the voting register may not actually cast ballots. Furthermore, voters who do cast ballots may abstain, deliberately voting for nobody, or they may spoil their votes, either accidentally or as an act of protest.
In the United Kingdom, the Electoral Commission distinguishes between "valid vote turnout", which excludes spoilt ballots, and "ballot box turnout", which does not.
A second problem with turnout measurements lies in the way turnout is computed. One can count the number of voters, or one can count the number of ballots, and in a vote-for-one race, one can sum the number of votes for each candidate. These are not necessarily identical because not all voters who sign in at the polls necessarily cast ballots, although they ought to, and because voters may cast spoiled ballots.
Demographics also have an effect. Older people tend to vote more than youths, so societies where the average age is somewhat higher, such as Europe; have higher turnouts than somewhat younger countries such as the United States. Populations that are more mobile and those that have lower marriage rates tend to have lower turnout. In countries that are highly multicultural and multilingual, it can be difficult for national election campaigns to engage all sectors of the population.