FACTBOX: Exit polls in the presidential race

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

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

* The exit poll helps news organizations predict 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, stationed at more than 1,300 polling locations nationwide, 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.

* 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 weigh 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 that might affect voter behavior.

* The survey is conducted by Edison Media Research and Mitofsky International. It is underwritten by the ABC, NBC, CBS, CNN and Fox News television networks and The Associated Press. The 2004 exit poll cost $10 million.

* Other news organizations can purchase the results of the survey for between $1,000 and $26,000. Researchers, advocacy groups and others may purchase the results one week after the election. (Reporting by Andy Sullivan; Editing by Doina Chiacu)