Election turnout is often cited as an indicator of the strength of the mandate of winning candidates, but it can be a misleading statistic: Turnout is usually measured as a proportion of registered voters rather than of those eligible to vote — and census numbers show that more than 70 million U.S. citizens of voting age are not registered voters.

Al Jazeera kept a close watch on voter turnout Tuesday, mindful of the possible impact of new voter ID laws in more than 30 states that critics feared would discourage or prevent voting by poorer Americans.

Turnout proved to be lower than previous years in all but 10 states, but the reasons for that decline are many: Some states lacked competitive races to draw voters to the polls; others cut polling hours or reduced early voting periods. And, in some states, new voter ID laws could have kept some voters away. Public opinion polls such as the one released by Gallup earlier this week suggested that fewer Americans cared about this election than in previous years.

But the data also proved more complicated than we anticipated. Election turnout is often cited as an indicator of the strength of the mandate of winning candidates, but it can be a misleading statistic: Turnout is usually measured as a proportion of registered voters, rather than of those eligible to vote – and U.S. census numbers show that more than 70 million U.S. citizens of voting age are not currently registered voters.

Turnout in midterm elections measured by participation of registered voters has has fallen since 1840, according to the Pew Research Center. Voter turnout in 2010 rose slightly, but 2014 turnout is a return to the declining trend.