FACTBOX - Exit polls in the U.S. presidential race

(Reuters) - When Americans choose a president on November 4, canvassers across the country will ask for whom they voted and what issues are most important to them.

Following are some facts about the Election Day "exit poll":

* The exit poll helps TV networks to determine the winner of an election, sometimes as soon as voting concludes in a state. In close races, analysts often wait for actual voting tallies to come in before making a call.

* The poll also asks voters about their top concerns and collects demographic data, which can highlight trends in the electorate.

* Canvassers will be stationed at more than 1,300 polling locations nationwide. They will collect questionnaires from more than 100,000 voters. The survey also includes 10,000 voters who cast absentee ballots or visited early-voting locations before Election Day.

* In order to avoid bias, canvassers stick to a pattern. They may ask every fifth voter they encounter to participate, or one in every 10, depending on the size of the location.

* Roughly half of all of those asked decline to participate. Canvassers note their age, race, sex and other demographic characteristics so analysts can make a reasonable guess about how they voted.

* Analysts also try to weight the results to make up for other discrepancies. In 2004, for example, more Democrats than Republicans were willing to participate.

* Analysts are sequestered to prevent leaks which might affect voter behaviour.