What kind of polls might journalists use to assess how voting has gone
during an election?

Typically each news organization works with a company that specializes in polling data. There is a science to the questions. They should be phrased in such a way that the question is neutral and not leaning towards one candidate over the other.

Some well know polling companies include: Harris, Gallup, Nielsen which are old-line public companies. Increasingly Universities are getting into the act as well, such as Siena College, Quinnipiac and Monmouth in conjunction with news organizations. Siena frequently is partnered with the NYT, Quinnipiac with TIME magazine and so on.

The polling organization makes sure that the polling places where they take the polls represent a true cross-section of the voting public. Taking polls only in rural areas or only densely populated city districts would skew the results. But no matter how carefully the questions are worded, or the cross-section chosen there is always a margin of error.

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